New guidelines for Allan-Herndon-Dudley syndrome: A rare disorder of thyroid hormone transport recommends lifelong monitoring by multiple specialists.

An international group of European endocrinologists and pediatric neurologists has developed a practical system for lifelong monitoring of patients with monocarboxylate transporter 8 deficiency, a rare genetic disorder also known as Allan-Herndon-Dudley syndrome. The guidelines are published in the journal Hormone Research in Paediatrics and are intended for children, adolescents, and adults.

This disease is unusual in that it simultaneously causes a deficiency of the hormone triiodothyronine in the brain and an excess of it in peripheral tissues. It is caused by mutations in the SLC16A2 gene, which encodes the specific thyroid hormone transporter MCT8. Without this transporter, triiodothyronine is poorly delivered to the central nervous system, while its blood concentration typically increases.

As a result, a single patient coexists with severe neurological impairments—severe developmental delay, hypotonia, dystonia, and spasticity—and signs of peripheral thyrotoxicosis: tachycardia, low body weight, muscle loss, accelerated bone turnover, and other complications. The disease is chronic, severe, and shortens life expectancy; according to the authors, approximately one in three patients dies in childhood.

The novelty of this publication lies not in the discovery of a new drug. The European Thyroid Association has already issued formal guidelines for diagnosis and treatment in 2024. This new study attempts to answer a more practical question: what exactly and how often should a patient's health be monitored throughout life to promptly detect cardiac, endocrine, neurological, nutritional, liver, and bone complications.

The main point of the new recommendations	What is offered
Disease	MCT8 deficiency/Allan-Herndon-Dudley syndrome
Gene	SLC16A2
Type of inheritance	X-linked
The main endocrine feature	High triiodothyronine with low/normal free thyroxine
The main neurological problem	Insufficient action of triiodothyronine in the central nervous system
The main peripheral problem	Thyrotoxicosis
Observation	Life imprisonment
Coordinator	Endocrinologist or neurologist
Key specialists	Neurology, endocrinology, cardiology, gastroenterology, orthopedics, nutrition, etc.
Specific treatment	Tiratricol in suitable patients
The main goal of management	Systematize monitoring of complications

[1]

What is MCT8 deficiency and why does the body find itself in a state of both "deficiency" and "excess" of the hormone?

MCT8 is a specialized protein that transports thyroid hormones across cell membranes. Its role is particularly important in supplying triiodothyronine to the central nervous system. In pathogenic variants of SLC16A2, this transporter function is partially or completely lost, resulting in insufficient triiodothyronine supply to the developing brain.

In the developing nervous system, thyroid hormones are essential for myelination, the formation of neural networks, synapses, and the normal maturation of brain structures. In MCT8 deficiency, delayed or impaired myelination, changes in the cortex and cerebellum, and impaired synaptic function, neurovascular complex, and astrocytes have been described. This helps explain the severity of the neurological phenotype.

But outside the brain, the situation is the opposite. Triiodothyronine can enter peripheral tissues via other transport systems. Its blood concentration is typically elevated in MCT8 deficiency, resulting in excessive hormonal stimulation of the liver, heart, bones, and other organs. This is what causes tachycardia, sweating, increased energy expenditure, low body weight, and accelerated bone turnover.

Therefore, the usual logic of "too little" or "too much" thyroid hormones doesn't apply here. In the brain, the hormone's effect is insufficient, while in peripheral tissues, it's excessive. It is this paradox that makes the disease difficult to treat: reducing excess triiodothyronine in the blood does not necessarily automatically correct its deficiency in the central nervous system.

Area of the body	What Happens with MCT8 Deficiency?
Brain	Insufficient triiodothyronine intake
Blood	Triiodothyronine is usually elevated
Heart	Impact of peripheral thyrotoxicosis
Liver	Enhanced thyroid signaling
Bones	Accelerated bone turnover
Muscles	Muscle loss may increase
General energy metabolism	Increased energy consumption

[2]

The disease often begins to be noticed at around four months.

Pregnancy and birth are uneventful for most children, and birth weight and length are typically consistent with gestational age. The first obvious problems often appear several months later, around four months of age. Parents and doctors notice pronounced muscle hypotonia, lack of normal head control, delayed motor development, and inadequate weight gain.

Later, the picture becomes more complex. Trunk hypotonia may persist, but dystonia and spasticity gradually develop in the limbs. Irritability, sleep disturbances, severe delays in intellectual and motor development, the persistence of primitive reflexes, and epileptic seizures are possible.

The symptoms themselves are not specific: many genetic, neurological, and metabolic diseases can produce similar symptoms. The authors therefore suggest caution when severe developmental delay is combined with a characteristic thyroid profile: elevated free triiodothyronine, low or low-normal free thyroxine, and normal or slightly elevated thyroid-stimulating hormone.

A simple additional indicator is the free triiodothyronine to free thyroxine ratio. The authors consider a value above 0.75 or above the 97th percentile highly suspicious for MCT8 deficiency. However, a definitive diagnosis must be confirmed genetically by identifying a pathogenic variant of SLC16A2.

Why routine newborn screening can miss disease

A unique feature of MCT8 deficiency is that standard neonatal screening for congenital hypothyroidism typically focuses on thyroid-stimulating hormone. However, in newborns with Allan-Herndon-Dudley syndrome, its concentration often remains within the reference range. Therefore, a child may successfully pass standard screening despite a pre-existing genetic hormone transport disorder.

Free triiodothyronine is not typically included in standard newborn screening programs. As a result, biochemical testing begins only after developmental delays, hypotonia, or poor weight gain have occurred. This contributes to diagnostic delays.

According to the authors of the guidelines, the median time between symptom onset and diagnosis was approximately 14 months. For an infant's rapidly developing nervous system, this delay can be significant, especially as therapeutic options emerge, the potential benefit of which may depend on early treatment.

The authors therefore draw attention to genetic panels. If a child undergoes genetic testing due to unexplained developmental delays or thyroid dysfunction, SLC16A2 should be included in the appropriate diagnostic panel. This potentially allows for a transition from years of diagnostic testing to earlier molecular confirmation of the disease.

Diagnostic stage	What to look for
Clinic	Hypotonia, developmental delay, poor head control
Triiodothyronine	Usually elevated
Free thyroxine	Low or low-normal
Thyroid-stimulating hormone	Normal or slightly elevated
FT3/FT4	>0.75 or >97th percentile is alarming
Confirmation	Pathogenic variant of SLC16A2
Routine neonatal TSH screening	May not detect the disease

[3]

The main innovation is that each patient should have one treatment coordinator.

The authors believe that one of the biggest problems with MCT8 deficiency is not a lack of specialists, but rather the fragmentation of care. A neurologist might focus on dystonia and epilepsy, an endocrinologist on triiodothyronine, a gastroenterologist on feeding, a cardiologist on tachycardia, and an orthopedist on scoliosis. Without a single coordinator, individual complications can easily slip through the cracks.

Therefore, immediately after diagnosis is confirmed, it is recommended to appoint a clinical lead physician responsible for overall coordination. Depending on the local healthcare system, this should most often be an endocrinologist or neurologist.

The leading physician is not obligated to treat all complications independently. Their role is to promptly involve a pediatric or adult neurologist, endocrinologist, cardiologist, gastroenterologist, orthopedist, dietitian, feeding specialists, rehabilitation specialists, and, in severe cases, the palliative care team.

For newly diagnosed patients, the authors propose using a dedicated monitoring checklist. The idea is quite practical: the rarity of the disease means that most doctors may only see one such patient in their entire career, so a systematic list of tests can reduce the likelihood of missing an important complication.

Thyroid tests are recommended to be done regularly, but triiodothyronine alone is not enough

Free triiodothyronine, free thyroxine, and thyroid-stimulating hormone remain the mainstay of endocrine monitoring. In the summary table, the authors suggest assessing thyroid parameters at baseline and approximately twice a year, increasing the frequency after the initiation of specific treatment and during dose adjustment.

However, a single triiodothyronine test does not always fully reflect the hormone's impact on organs. Its concentration fluctuates throughout the day, and its half-life is relatively short. Therefore, researchers suggest supplementing the standard panel with markers of thyroid signaling in specific tissues, if necessary.

For example, bone-specific alkaline phosphatase may reflect the effects of thyroid hormones on the skeleton, while electrocardiogram parameters may reflect their effects on the heart. Sex hormone-binding globulin, ceruloplasmin, and copper may be used for liver thyroid signaling. Estimated values for peripheral deiodinase activity and thyroid secretory capacity are also studied.

The authors specifically warn that some of these markers are still research or surrogate markers, especially in children. For many, there are no well-validated age references or minimal clinically significant changes. Therefore, the new system does not propose mechanically manipulating the numbers; physicians must evaluate the dynamics of a specific patient relative to their own baseline.

What do they control?	Possible meaning
Free triiodothyronine	The main marker of peripheral thyrotoxicosis
Free thyroxine	Complements the thyroid profile
Thyroid-stimulating hormone	Evaluation of axis regulation
Sex hormone-binding globulin	Thyroid action in the liver
Ceruloplasmin / copper	Additional peripheral markers
Bone alkaline phosphatase	Impact on bone
Electrocardiogram	Effect on the heart
Alanine aminotransferase	Liver condition
Creatinine	May reflect peripheral effects and muscle mass

[4]

Tiratricol became the first drug approved in Europe for the control of peripheral thyrotoxicosis

The most significant recent development in the drug's development was the introduction of tiratricol, or TRIAC. The European Medicines Agency approved Emcitate as an orphan drug for patients with MCT8 deficiency in February 2025.

Tiratricol is a thyroid hormone analogue capable of bypassing MCT8-dependent transport. It binds to thyroid receptors, particularly the beta receptor, reducing circulating triiodothyronine and ameliorating the manifestations of peripheral thyrotoxicosis. Among thyroid analogues, it currently has the most compelling clinical evidence.

However, the authors emphasize a fundamental limitation: evidence for improved neurological outcomes is significantly weaker. The fact that the drug reduces thyrotoxicosis in the heart, liver, and other peripheral tissues does not guarantee restoration of damaged brain development. This is especially important when discussing treatment expectations with patients' families.

The European Thyroid Association also considers diiodothyropropionic acid as a possible hormone analogue, but its evidence base is weaker. Conventional antithyroid drugs such as carbimazole or methimazole are not recommended for MCT8 deficiency: the problem is not a simple overproduction of thyroid hormone by the thyroid gland, and such treatment may further worsen the central hormonal deficiency.

When treated with tiratricol, a standard T3 test can be misleading to the physician.

A new publication highlights a rather technical, yet clinically important, issue. Many standard laboratory immunoassays for total and free triiodothyronine can partially "see" the thyrotropin molecule as T3. As a result, the laboratory may report artificially elevated triiodothyronine concentrations.

This is especially dangerous during treatment selection. If the doctor decides that T3 is still too high, even though part of the signal is actually generated by the drug itself, this could potentially lead to misinterpretation of the response and unnecessary changes in therapy.

Therefore, the authors recommend, whenever possible, measuring triiodothyronine using liquid chromatography with mass spectrometry. This method allows for better separation of intrinsic T3 from triiodothyronine. If this method is unavailable, it is advisable to engage a laboratory experienced in the relevant immunoassays and their cross-reactivity.

This is a good example of why treating a rare disease requires not only a drug but also specialized infrastructure. The same T3 value obtained by two different laboratory methods may have different meanings in a patient taking tiratricol. Therefore, it is impossible to interpret the test without taking the method of measurement.

It is proposed that neurological monitoring be made as individual as possible

Hypotonia, dystonia, and spasticity are the central manifestations of the disease. They significantly limit motor function and quality of life. Therefore, neurological care should include not only a diagnosis but also long-term monitoring of muscle tone, movement, development, and the patient's ability to perform activities.

For children, developmental and gross motor scales can be used, including the Bayley Scales of Infant and Toddler Development and the Gross Motor Function Measure. However, for patients with extremely severe developmental disabilities, it is often only possible to complete certain elements of the standard scale.

The authors also highlight the human side of monitoring. Repeated testing of a child who, due to illness, is barely showing expected development can create an additional emotional burden for the family. Therefore, the objectivity of measurements must be balanced against the actual benefits of the test and the burden on parents.

Magnetic resonance imaging (MRI) often reveals delayed or insufficient myelination, but repeat imaging should not be performed automatically once a diagnosis is established. The authors suggest MRI only be performed when there is a specific clinical question, as the examination can be difficult for a patient with significant disability and, by itself, will not necessarily change treatment.

Seizures require special attention

In some patients, epileptic seizures become more noticeable with age. Severe movement disorders can make it difficult to distinguish epileptic events from dystonic movements, stereotypies, or other manifestations of the underlying disease.

The authors suggest starting with a detailed medical history and family observations. Particular attention should be paid to suspicious nocturnal activity, episodes of sudden "blackouts," or repetitive movements. Short video recordings made by relatives on their phones can help the doctor understand the nature of the episode.

Electroencephalography (EEG) is recommended initially and then as clinically indicated, rather than automatically at uniform intervals for all patients. This approach reflects one of the key ideas of the new guidelines: minimizing tests that don't answer a specific clinical question.

When prescribing antiepileptic drugs, liver function must be additionally considered. Some anticonvulsants are potentially hepatotoxic, and in patients with MCT8 deficiency, liver markers may already be altered due to increased thyroid signaling. Therefore, liver function monitoring in this situation requires more careful monitoring.

The heart has become one of the key objects of constant monitoring

Excessive triiodothyronine action can increase heart rate, alter blood pressure, cardiac output, and cardiac electrical conductivity. Therefore, long-term peripheral thyrotoxicosis can create significant cardiovascular stress.

A particularly alarming fact is that sudden death is among the leading causes of death in MCT8 deficiency and may be associated with cardiac events. A large retrospective cohort identified resting tachycardia, premature atrial contractions, and elevated blood pressure.

Moreover, most patients diagnosed with cardiac abnormalities had no known prior cardiac history. The authors interpret this as a possible sign of underdiagnosis and an argument in favor of an active baseline cardiac examination.

An electrocardiogram and echocardiogram are recommended initially and then as clinically necessary. In some cases, 24-hour monitoring is helpful. It is recommended to monitor not only the heart rate but also the QT, JT, T-peak-T-end intervals, and the presence of extrasystoles.

What to control	When
Electrocardiogram	Initially, then according to indications
Echocardiography	Initially, then according to the clinical situation
Blood pressure	Regularly
24-hour monitoring	If necessary
Pulse rate	As part of ongoing surveillance
QT/JT/T-peak-T-end	When analyzing an electrocardiogram

[5]

Nutrition turned out to be not just an adjunct, but a potentially vital part of treatment.

Feeding problems are among the most common complications of the syndrome. Oropharyngeal dysphagia, gastroesophageal reflux, and poor weight gain increase the risk of aspiration, malnutrition, and respiratory infections.

However, specialized enteral nutrition is not used by all those potentially in need. In an international registry, 17 of 33 surveyed families reported feeding problems, but among the 16 children with significant nutritional deficiencies, only two were fed via gastrostomy.

Low body weight in MCT8 deficiency is caused by several mechanisms: peripheral thyrotoxicosis increases basal metabolic rate, impaired swallowing reduces food intake, and muscle hypotonia and subsequent atrophy further worsen the condition. As a result, the patient's physiological reserve is reduced.

The relationship with prognosis is particularly important. Underweight children aged 1-3 years had a significantly higher risk of death, and respiratory infections—one of the leading causes of death in this group—were more common in malnourished patients. Therefore, the authors recommend considering early nutritional support as part of complication prevention, not just as a way to increase weight.

Gastrostomy is suggested to be discussed earlier if swallowing becomes unsafe.

In cases of persistent dysphagia, recurrent aspiration, or failure to provide adequate oral nutrition, the authors recommend early consideration of a gastrostomy or jejunostomy. The goal of such an intervention is not only to deliver calories but also to reduce the risk of aspiration pneumonia and facilitate safe medication administration.

The decision should not be made automatically based on a single weight figure. It should be discussed with parents or other caregivers, taking into account swallowing patterns, infections, nutritional status, quality of life, and the family's resources.

The management of such patients requires the involvement of more than just a gastroenterologist. Ideally, a neurologist, a nutritionist, a speech therapist or swallowing specialist, and a gastroenterologist are needed, as the cause of the problem is usually multifactorial.

The summary chart suggests assessing nutritional status at baseline and approximately twice a year, and monitoring children's weight during regular pediatric examinations. If signs of malnutrition are detected, it is also advisable to assess vitamin status.

The liver requires monitoring both due to disease and medications.

In some patients, transaminases and other liver markers are elevated. This may reflect the increased effect of thyroid hormones on the liver, as the peripheral organs are subject to excessive thyroid stimulation.

Polypharmacy creates an additional complication. Patients may simultaneously receive antiepileptic drugs, muscle tone control medications, and other medications, some of which can damage the liver. Therefore, liver enzyme abnormalities cannot be automatically attributed to the syndrome itself.

In an open-label study of tiratricol, 25 of 46 patients, or 54%, already had mild elevations of alanine aminotransferase, aspartate aminotransferase, or gamma-glutamyltransferase at baseline. The authors attributed this, in part, to the concomitant use of potentially hepatotoxic drugs.

Therefore, it is recommended to assess liver function before initiating therapy and monitor it especially closely when combining several medications with potential hepatotoxicity. This is not to say that tiratricol necessarily damages the liver, but rather to consider the complex underlying biochemical background of these patients.

Bone tissue can suffer for two reasons at once.

The first mechanism is chronic exposure to elevated triiodothyronine. Excess thyroid hormones can accelerate bone turnover, reduce bone mineral density, and increase the risk of osteoporosis and fractures.

The second factor is severe limited mobility. Many patients place little or no mechanical load on the skeleton, which is necessary for bone tissue maintenance. Therefore, hormonal and mechanical factors can act simultaneously.

The authors suggest conducting the first bone mineral density measurement using dual-energy X-ray absorptiometry beginning at puberty. If the clinical situation requires a repeat measurement, the approximate interval could be approximately 3-5 years, but a fixed schedule is not suitable for everyone.

Calcium and vitamin D intake should be assessed separately. If intake is inadequate or a laboratory-confirmed deficiency is present, supplements may be prescribed. Thus, fracture prevention encompasses a combination of endocrine monitoring, nutrition, and orthopedic care.

Why the guidelines no longer apply only to children

Historically, Allan-Herndon-Dudley syndrome was primarily considered a childhood neuroendocrine disorder due to its early onset and high infant mortality. However, an increasing number of patients are surviving into adolescence and adulthood.

This creates a new challenge—the transition between pediatric and adult medicine. Pediatric neurologists and pediatric endocrinologists are familiar with the patient's history, but after the patient reaches adulthood, care may shift to new specialists who are significantly less likely to encounter MCT8 deficiency.

Therefore, the new publication deliberately covers the entire life cycle. Some complications become more noticeable in childhood, while others—such as osteoporosis or the long-term cardiovascular consequences of thyrotoxicosis—acquire additional significance in adolescents and adults.

The authors essentially propose abandoning the concept of the disease as a static, congenital disability. The clinical picture changes with age, so the intensity and focus of monitoring should also change. This is one of the reasons why they consider a rigid, uniform algorithm for all patients inappropriate.

One in three patients may die in childhood.

One of the most devastating figures cited by the authors is that approximately one-third of patients do not survive childhood. This emphasizes that the syndrome is not only a cause of severe neurological disability but also a condition that directly impacts survival.

The causes of death are multifactorial. Respiratory infections, which can be contributed to by swallowing difficulties, aspiration, and exhaustion, play a significant role. Sudden death, potentially associated with cardiovascular complications of thyrotoxicosis, is a particular cause.

This is why the authors focus on more than just neurological development. Even if severe damage to the central nervous system cannot be completely reversed, active treatment of peripheral thyrotoxicosis, nutritional support, aspiration prevention, and cardiac monitoring can potentially impact somatic complications and quality of life.

However, the new guidelines themselves are not a study demonstrating a reduction in mortality thanks to the proposed surveillance system. They formulate a practical strategy based on accumulated data and expert experience. The effectiveness of the entire monitoring system as a unified intervention remains to be tested.

How reliable are the recommendations themselves?

This is an important question, as the publication is called a guideline, but its methodological basis differs from major guidelines for common diseases. The document was developed by five specialists—two pediatric neurologists, two pediatric endocrinologists, and one endocrinologist—following a virtual expert meeting on November 26, 2024.

The authors then conducted a targeted literature search of PubMed. Initially, 873 publications were identified; after the initial screening, 234 papers were reviewed using abstracts, and the full text was downloaded for 97 publications. Based on the literature and clinical experience, the expert group refined the recommendations.

Moreover, the evidence was not assessed using the GRADE system, and much of the literature on such a rare disease consists of small cohorts, cell and animal studies, computer models, and isolated observations. The authors openly cite this as one of the main limitations of the document.

The feasibility of implementing the entire proposed monitoring system in routine healthcare systems has not yet been verified. The availability of molecular diagnostics, mass spectrometry, reference centers, and tiratricol varies significantly between countries. Therefore, these recommendations should be considered primarily as an expert practical framework that requires further prospective validation.

Strengths	Restrictions
Systematization of lifelong observation	A very rare disease
Association of Neurology and Endocrinology	Few large studies
873 publications found	The GRADE system was not applied
97 full-text papers were reviewed	Part of the evidence is small series and experimental data
Specific monitoring parameters	The feasibility of the scheme has not yet been verified.
Children and adults are taken into account	Availability of screenings varies by country

[6]

Funding and potential conflicts of interest require special attention.

The development of the guidelines began with an expert meeting organized by Egetis Therapeutics, the manufacturer of Emcitate, or tiratricol, approved for the treatment of peripheral thyrotoxicosis in MCT8 deficiency. The company funded the authors' participation in the expert panel.

Egetis also funded editorial assistance, author comments, and publication costs. Medical assistance in manuscript preparation was provided by Prescript Communications Ltd, at Egetis' expense. The authors state that the company had no role in formulating recommendations, interpreting data, writing the final text, or the decision to publish.

Johannes Dietrich reports funding or personal fees from several companies, including Egetis, Novo Nordisk, Abbott, Medtronic, Pfizer, and others. Ilja Dubinski reports honoraria from Egetis, Merck, Sandoz, Kyowa Kirin, and several other organizations. The remaining authors declare no conflicts of interest.

The presence of commercial funding does not automatically invalidate the recommendations: a significant portion of the guidelines concern cardiology, nutrition, neurology, bone tissue, and care management, rather than the drug itself. However, since the manufacturer of the only specific drug approved in the European Union directly funded the expert committee and publication support, this fact must be taken into account when evaluating the document, particularly its medicinal sections.

What the new guidance recommends monitoring

System	The main problem	Monitoring
Thyroid gland	Peripheral thyrotoxicosis	FT3, FT4, TSH and additional markers
Nervous system	Hypotension, dystonia, spasticity, epilepsy	Clinical presentation, EEG according to indications, developmental scales
Brain	Hypomyelination and other changes	MRI only when clinically necessary
Heart	Tachycardia, conduction disturbances, pressure	ECG, echocardiography, sometimes Holter
Nutrition	Underweight, dysphagia	Dietitian assessment, body weight
Swallowing	Aspiration	Specialized assessment, gastrostomy discussion
Gastrointestinal tract	Reflux	Clinical monitoring and treatment
Liver	Changes in enzymes, medications	ALT, AST and other indicators
Bones	Osteoporosis and fractures	DEXA since puberty
Orthopedics	Scoliosis, hip subluxation	Examination and visualization as indicated
Family	High care load	Psychological and social support

[7]

What does publication mean in practice?

The main change is not the emergence of a new diagnostic test, but the shift from piecemeal treatment of individual symptoms to lifelong management of the disease as a unified neuroendocrine system. Patients are encouraged to undergo regular assessments not only by a neurologist and endocrinologist, but also for cardiac, nutritional, swallowing, liver, and bone health.

The second principle is establishing baseline values early. Since the disease has enormous individual variability and there are no well-validated minimum clinically significant changes for most parameters, a patient's own historical dynamics may be more useful than comparison with a single, universal number.

The third principle is not to examine for the sake of examination. For example, magnetic resonance imaging may reveal a characteristic myelination disorder, but the authors do not recommend repeating it regularly without a specific clinical question. The same applies to a number of complex procedures that place a significant burden on a severely disabled patient and their family.

Finally, the advent of tiratricol makes systematic laboratory and clinical monitoring even more important. The ability to reduce peripheral thyrotoxicosis represents a significant therapeutic advance, but the neurological effect remains less certain, and laboratory measurement of T3 during therapy presents its own technical challenges. Therefore, the new guidelines mark the beginning of an era of more organized care rather than a complete solution to the syndrome's problems.

What the manual claims - and what it doesn't prove

Correct conclusion	What the article did not prove
Patients need lifelong multidisciplinary care	That a specific regimen has already been proven to increase life expectancy
Early diagnosis is important	That neonatal screening has already been implemented
FT3 is usually elevated	That one test is enough for a diagnosis
Genetic testing of SLC16A2 confirms the diagnosis	That all gene variants have the same severity
Tiratricol reduces peripheral T3	That it is guaranteed to restore neurological development
Feeding and body weight are critically important	That a gastrostomy is necessary for every patient
Cardiac complications may be underdiagnosed	That all sudden deaths have a cardiac cause
Bone density should be monitored	That every patient will definitely get osteoporosis
Monitoring should be individualized	That there is one universal scheme for everyone

Results

MCT8 deficiency is a rare example of a condition in which the same hormone is simultaneously too weak in the brain and too strong in peripheral tissues. This duality explains the combination of severe neurological disability with tachycardia, exhaustion, bone problems, and other signs of thyrotoxicosis.

The new guidelines recommend starting with early genetic confirmation of the diagnosis and immediately assigning a lead physician to coordinate a multidisciplinary team. Thyroid levels and nutrition should be monitored regularly; the heart, bones, liver, and nervous system should be monitored according to a pre-designed, individualized program.

Warnings about malnutrition and cardiovascular complications are especially important. Feeding problems are common, but gastrostomy feeding is not used for everyone who needs it, and cardiac problems can be detected in patients with no previous cardiac diagnosis. These are areas where systemic monitoring has the potential to change clinical practice even now.

However, the recommendations remain an expert framework for an ultra-rare disease: the evidence base is limited, the GRADE system has not been applied, and the effectiveness of the proposed monitoring regimen has not yet been tested in prospective studies. Therefore, the authors emphasize the individual trajectory of each patient rather than a rigid algorithm.

News source

Dietrich JW, Jelesch E., Linder-Lucht M., Völkl TMK, Dubinski I. Clinical and Biochemical Monitoring of Monocarboxylate Transporter 8 Deficiency (Allan-Herndon-Dudley Syndrome) Across the Lifespan: Practical Considerations for Multidisciplinary Care. Hormone Research in Paediatrics. 2026. Published online April 2, 2026.

This publication is classified by PubMed as a Practice Guideline. It is designed as a practical supplement to the 2024 European Thyroid Association guidelines and is primarily focused on long-term clinical and biochemical monitoring of patients with MCT8 deficiency.

DOI: 10.1159/000551857.