This therapeutical trial will be conducted in patients with the Allan-Herndon-Dudley Syndrome (AHDS), which is mutations in MCT8.

MCT8 is a thyroid hormone (TH) transporter which is crucial for the transport of TH from the blood into different tissues. Dysfunction of MCT8 results in a lack of TH (hypothyroidism) in tissues that depend on MCT8 for TH uptake. This local hypothyroidism in the brain of these patients causes severe psychomotor retardation.

In addition, TH serum parameters are highly abnormal in AHDS: high T3, low T4 and normal TSH levels. The high serum T3 levels cause local hyperthyroidism in tissues that do not depend on MCT8 for cellular transport of TH, resulting in a low body weight and reduced muscle mass.

Currently, no adequate treatment is available for the AHDS. A T3 analog that does not depend on MCT8 for its cellular entry could, at least partially, restore the abnormalities found in AHDS. Several in vivo, in vitro and animal studies have shown that the T3 analog Triac is a very promising candidate:

1. Triac binds to the same TH receptors as T3;
2. Cellular uptake of Triac does not depend on functional MCT8. Hence, in AHDS patients Triac will also be available in tissues that require functional MCT8 for TH uptake, e.g. the brain;
3. In vitro studies have shown that neuronal cells differentiate equally well in the presence of either Triac or T3;
4. In Mct8 deficient mice, Triac is taken up by the brain and suppresses serum TSH levels; consequently, serum T3 and T4 levels were lowered;
5. Triac is the treatment of choice in patient with the resistance to thyroid hormone (RTH) syndrome. Patient with RTH have high serum TSH and thyroid hormone levels, which shows strong similarities to the profile found in AHDS patients; the longstanding experience with Triac in RTH indicates its safety and tolerability .

Thus, Triac treatment could result in normalization of the abnormal serum TH values in AHDS patients. Furthermore, Triac could replace the function of T3 in tissues that depend on MCT8 for TH uptake (e.g. brain).

The current trial will investigate if Triac treatment in ADHS patients

1. reduces the toxic effects of the high T3 levels
2. restores the local TH deficiency in brain.

All patients were treated with Triac (Téatrois tablets 350 microgram, Rare Thyroid Therapeutics) by individualized dose-escalation, following a pre-defined dose-escalation protocol. After the initial dose of Triac (350 microgram) was administered and no predefined dose-limiting toxicities were observed, the daily dose was increased progressively in 350 microgram steps, with a goal of attaining serum total T3 concentrations within the target range of 1·4-2·5 nmol per liter. The maintenance Triac dose was continued throughout the rest of the study period, but could be further adjusted according to the dose-escalation protocol if T3 concentrations were outside the target range during control visits.

Patients were assessed for study outcomes at baseline and 12 months after starting Triac administration. In the interval, patients were evaluated and screened for clinical and biochemical signs of hypothyroidism or hyperthyroidism, adverse events were recorded and adherence to therapy was assessed. All study procedures were specified in standard operating procedures, and were performed by well-trained investigators. Neuropsychological tests were conducted according to their manual. All biochemical measurements were performed in a central laboratory (Erasmus Medical Centre). To account for any interference of Triac in the measurement of serum T3 concentrations, conventional methods were employed to correct for cross-reactivity.

Inclusion Criteria:

• clinically relevant mutation in the MCT8 gene, resulting in the clinical phenotype of AHDS.

Exclusion Criteria:

• Major illness or recent major surgery (within 4 weeks) unrelated to AHDS
• Patients who are participating in ongoing RCTs of therapeutic interventions (including clinical trials of investigational medicinal products);
• Known allergy to components in Triac tablets;
• Patients that have any contra-indication for Triac treatment.