Peripheral resistance to thyroid hormone in an infant.

Peripheral resistance to thyroid hormone, a syndrome characterized by elevated serum total and free thyroid hormone levels and abnormal TSH suppression without manifestations of hyperthyroidism, was studied in a clinically euthyroid 6-month-old infant. Initial serum concentrations of T4, T3, and TSH were 22.1 micrograms/dl, 334 ng/dl, and 7.6 microunits/ml, respectively; infusion of synthetic TRH increased the serum TSH to 47.4 microunits/ml, an exaggerated response. Pituitary insensitivity to T3 was investigated by measuring these parameters in response to consecutive 7-day courses of increasing doses of T3. Four times the calculated replacement dose of T3 (40 micrograms/day) was required to normalize the serum T4 and the serum TSH response to TRH. After administration of 80 micrograms/day T3, the serum TSH response to TRH was virtually abolished, but no clinical signs of thyroid hormone excess were observed. High doses of T4 blunted the serum TSH response to TRH in a manner similar to T3. Prednisone also decreased the TSH response to TRH but had no effect on serum thyroid hormone concentrations. In an attempt to determine the mechanism of thyroid hormone resistance, specific nuclear T3 binding was compared in cultured skin fibroblasts from the patient and a normal infant. Normal fibroblast nuclei had a single binding site with a Ka of 3.1 X 10(9) M-1. In contrast, the Scatchard plot of the patient's T3 binding was curvilinear, compatible with a high affinity site that had a Ka (4.2 X 10(9) M-1) similar to that of the normal fibroblasts and a second low affinity site (Ka = 2.7 X 10(8) M-1). Supraphysiological concentrations of T3 elicited a dose-related increase in fibroblast glucose consumption, which was similar in cells from both the patient and from a normal infant. In conclusion, pituitary and peripheral resistance to thyroid hormone has been demonstrated in this infant, but despite the abnormality of nuclear T3 binding, the cellular mechanisms remain unclear.