Thyrotrophin in rat and human serum after withdrawal of carbimazole

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Abstract
In rats and humans receiving antithyroid drugs, a rebounding radioiodine uptake occurs after withdrawal of the drug even if the thyrostatic agent had not been given long enough to decrease serum protein -bound iodine (PBI). The mechanism of this phenomenon is controversial. In this paper, data or rat ex-periments are first presented which show that: the thyrotrophin (TSH) content of the blood increases to high levels within 6 days of carbimazole treatment; the rebound of thyroidal radioiodine uptake which begins following discontinuation of the tyrostatic agent dis-appears at the same rate as blood TSH concentration returns to control levels; and the TSH content of serum from carbimazole-treated rats rises to nigh levels well before a decrease in PBI becomes measurable. Possible explanations are sought to bring this latter finding in line with the concept of feedback control. Experiments in humans similar to those performed in rats did not yield conclusive results, possibly because of inadequate sensitivity of the TSH bio-assay used in this work. The basic considerations for a proposed procedure to demonstrate normal pituitary TSH reserve by means of an overshooting radioiodine clearance following short-term carbima-zole traetment are probably valid, although tracer techniques may give false negative results in the presence of high iodine intake.