CONTEXT: Monocarboxylate transporter 8 (MCT8) is a thyroid hormone-specific cell 
membrane transporter. MCT8 deficiency causes severe psychomotor retardation and 
abnormal thyroid tests. The great majority of affected children cannot walk or 
talk, and all have elevated serum T(3) levels, causing peripheral tissue 
hypermetabolism and inability to maintain weight. Treatment with thyroid hormone 
is ineffective. In Mct8-deficient mice, the thyroid hormone analog, 
diiodothyropropionic acid (DITPA), does not require MCT8 to enter tissues and 
could be an effective alternative to thyroid hormone treatment in humans.
OBJECTIVE: The objective of the study was to evaluate the effect and efficacy of 
DITPA in children with MCT8 deficiency.
METHODS: This was a multicenter report of four affected children given DITPA on 
compassionate grounds for 26-40 months. Treatment was initiated at ages 8.5-25 
months, beginning with a small dose of 1.8 mg, increasing to a maximal 30 mg/d 
(2.1-2.4 mg/kg · d), given in three divided doses.
RESULTS: DITPA normalized the elevated serum T(3) and TSH when the dose reached 
1 mg/kg · d and T(4) and rT(3) increased to the lower normal range. The 
following significant changes were also observed: decline in SHBG (in all 
subjects), heart rate (in three of four), and ferritin (in one of four). 
Cholesterol increased in two subjects. There was no weight loss and weight gain 
occurred in two. None of the treated children required a gastric feeding tube or 
developed seizures. No adverse effects were observed.
CONCLUSION: DITPA (1-2 mg/kg · d) almost completely normalizes thyroid tests and 
reduces the hypermetabolism and the tendency for weight loss. The effects of 
earlier commencement and long-term therapy remain to be determined.