Serum thyroid hormone concentrations and recovery of TSH secretion after excision of autonomously functioning thyroid nodules

Abstract

Serum concentrations of TSH, TT 4, TT 3 and rT 3 were monitored for one month after excision of ten autonomous thyroid adenomas which had suppressed TSH secretion. Basal serum TSH levels start to increase between 30 hr and 20 days after surgery reaching normal and steady levels by the 24 day. Serum TT 3 concentrations rapidly decrease in the first 20 hr to values near the lower limit of the normal range. Thereafter TT 3 levels change little until pituitary TSH secretion recovers. Serum TT 4 levels also fall, but much more slowly than TT 3. Rising TSH levels stimulate residual extranodular thyroid tissue secretion of both TT 3 and TT 4. However, serum TT 3 levels rise more rapidly than TT 4 levels. Preoperative serum concentrations of both TT 4 and TT 3 are related directly to the time required until the beginning of the postoperative rise in TSH levels, and inversely with the maximal postoperative serum TSH concentration achieved. Throughout the study period, for all patients, serum TSH concentrations were inversely related to serum TT 4 concentrations. These data suggest that, although the time required until the onset of recovery of TSH secretion is directly related to preoperative levels of TT 4 and TT 3, the regulation of postoperative TSH levels is dependent upon serum TT 4 levels. Also, the serum TT 3 levels constant at the lower limit of the normal range before recovery of TSH secretion, and the preferential rise in serum TT 3 concentrations associated with rising TSH secretion, may prevent or abbreviate temporary postoperative hypothyroidism.