In 58 patients treated with radioiodine for hyperthyroidism 5–22 yr previously levels of serum endogenous thyrotropin (TSH) were elevated in about 50% of patients with normal serum protein-bound iodine (PBI) and in about 20% of patients with normal serum PBI treated with thyroid. The levels of serum PBI were normal in about 25% of patients with elevated serum TSH and in about 50% of patients with elevated serum TSH on thyroid. Of 33 patients with elevated serum TSH, 31 did not respond to exogenous TSH. Of 11 euthyroid patients with normal serum PBI and high serum TSH, 4 had developed low serum PBI 3 yr later, whereas others continued normal 19–41 months later. Of 21 patients with low serum PBI and high serum TSH given thyroid, none of those who were euthyroid and all of those who were hypothyroid responded clinically; those who could not be classified clinically as either euthyroid or hypothyroid varied in response. There is good correlation between high levels of endogenous serum TSH and lack of response to exogenous TSH; but often patients have high endogenous serum TSH with a) normal serum PBI, b) euthyroidism remaining for years, or c) no clinical response to thyroid extract.