Suppression of Pituitary TSH Secretion in the Patient with a Hyperfunctioning Thyroid Nodule

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Abstract

10 patients with a single hyperfunctioning thyroid nodule each were studied for pituitary thyrotropin (TSH) suppression. They were judged to be euthyroid on clinical grounds. The total thyroxine (T4D), free thyroxine (FT4), total triiodothyronine (T3D), and free triiodothyronine (FT3) were normal in most of the patients. Incorporation of 131I into the hyperfunctioning thyroid nodules was not suppressed by the administration of physiological doses of T3. Basal serum TSH concentrations were undetectable (<0.5 - 1.0 μU/ml) in all patients. The metabolic clearance of TSH in one patient before and after excision of the thyroid nodule was unchanged (40 vs. 42 ml/min) whereas the calculated production rate was undetectable before the operation (<29 mU/day) and normal after (103 mU/day). These data, in one patient, suggest that the undetectable concentration of TSH in these patients is a result of suppressed TSH secretion rather than accelerated TSH clearance.

Authors

E. C. Ridgway, B. D. Weintraub, J. L. Cevallos, M. C. Rack, F. Maloof