Dextrothyroxine in the Treatment of Generalized Thyroid Hormone Resistance in a Boy Homozygous for a Defect in the T3 Receptor

The dextroisomer of thyroxine (D-T 4 ) has been shown to have suppressive effects on pituitary TSH secretion in euthyroid individuals and patients with mild thyroid hormone resistance. We treated a 3-year-old boy with D-T 4 who was homozygous for a T 3 receptor defect, resulting in a complex clinical picture of tissue-specific hyperthyroidism and hypothyroidism. There was no evidence of significant alteration in thyroid physiology, including serum concentrations of basal and TRH stimulated TSH or echocardiographic parameters measuring systolic time interval. We conclude that D-T 4 at a daily dose of 6 mg (0.65 mg/kg) was ineffective in this boy with homozygous dominant negative thyroid hormone resistance.