Basal serum TSH and the peak TSH response to a 500 μg TRH bolus were measured in 57 euthyroid and in 29 hypothyroid subjects either receiving graded thyroid hormone replacement or acutely removed from full replacement therapy. Serum TSH, total T4 and T3 were determined by sensitive radioimmunoassay methods. The peak versus basal TSH data for hypothyroid patients were linear within individuals. The regression slope of the peak versus basal TSH data for all hypothyroid subjects did not differ significantly from the corresponding slope for all euthyroid subjects. Basal and peak TSH versus T3 and T4 data for hypothyroid patients were also linear within each individual. Moreover, the regression of the basal TSH values averaged over the non-replacement to full replacement state against the TSH versus T3 slope had a significant negative correlation. This trend leads to an array of regression lines which average to the familiar hyperbolic relationship between thyrotrophin and thyroid hormone levels in man.