Eight patients with thyroid hormone resistance were found in four generations of a kindred containing 19 members. Results of studies in this family are consistent with an autosomal dominant mode of inheritance for this disorder. The affected family members were clinically euthyroid but all had goiters and markedly increased serum thyroid hormone levels: thyroxine (T4) = 21.1 +/- 2.1 microgram/dl; triiodothyronine (T3) = 323 +/- 60 ng/dl; free T4 = 5.4 +/- 0.9 ng/dl; and free T3 = 1,134 +/- 356 pg/dl (mean +/- SD). Serum thyrotropin (TSH) levels were normal or slightly elevated in six patients and responded normally to the administration of thyrotropin-releasing hormone (TRH) and L-triiodothyronine. Two patients who had previously undergone subtotal thyroidectomy had elevated baseline serum TSH levels and exaggerated TSH responses to the administration of TRH suggesting subclinical hypothyroidism despite elevated total and free thyroid hormone levels. The absence of thyrotoxicosis and normal serum TSH levels despite elevated serum free T3 and T4 levels in the untreated members of this family are consistent with resistance of pituitary and peripheral tissues to the actions of thyroid hormones. In addition, the absence of hypothyroidism and normal responsiveness of serum TSH to TRH and L-triiodothyronine administration in untreated family members suggest that the thyroid has compensated for the hormone resistance by increased secretory activity under the control of pituitary TSH secretion.