Serum Thyrotropin (TSH) Levels after Thyroid Ablation Compared with TSH Levels after Exogenous Bovine TSH: Implications for131I Treatment of Thyroid Carcinoma

In a group of ten patients who had undergone thyroid ablation for thyroid carcinoma, serum endogenous TSH was measured after withdrawal of triiodothyronine treatment. Serum TSH levels rose progressively during the 10–20 days of observation. Only two patients reached a plateau in their TSH level 10 and 15 days after withdrawal. Another group of ten subjects (5 euthyroid, 5 hypothyroid) were given 10 U bovine thyrotropin im on 3 successive days. Bovine TSH levels in their serum measured in a specific radioimmunoassay reached a peak of approximately 100–120 μU/ml in 2–4 hr and declined about 50% at 8–10 hr. Successive injections did not have an additive effect on the level achieved. Assuming the units of human and bovine TSH have an equivalent effect in man, the sustained elevated levels reached 2–3 weeks after withdrawal of triiodothyronine probably cause as much stimulation of residual functional thyroid tissue as does administration of bovine thyrotropin.