Recovery of initially suppressed thyroidal radioiodine uptake during long-term antithyroid treatment of euthyroid volunteers and hyperthyroid patients.

Abstract

ABSTRACT Carbimazole in daily doses of either 15 or 45 mg acutely depressed the thyroidal 131I-uptake (measured at 2, 8 and 24 hours after administration of the tracer) in 34 healthy human volunteers. Fourteen of these subjects were kept on carbimazole for four weeks. A gradual recovery of the radioiodine uptake was observed in every individual. By the end of the fourth week the 24 h uptake had risen by 100% over the value observed with acute suppression in the 15 mg carbimazole group (i. e. from 5.7 to 11.4%) and by 500% in the 45 mg carbimazole group (i.e. from 2.7 to 15.8%). The corresponding figures for the 8 h uptake were 100 to 250% and for the 2 h uptake 40 to 60%. In three individuals the uptake in the fourth week of carbimazole treatment was distinctly higher than the pretreatment control values. Thus the acute suppression of a complete blocking dose of carbimazole on thyroidal radioiodine uptake is partially or even totally overcome within a few weeks of chronic treatment. It is therefore possible, in human subjects, to produce experimentally a high thyroidal radioiodine uptake by antithyroid drug treatment alone, in the absence of concomitant severe iodine deficiency. This finding may have a bearing on the interpretation of radioiodine studies carried out on endemic goitre. Comparable findings were obtained in 23 hyperthyroid patients treated with carbimazole for a mean of 28 weeks. In order to elucidate the mechanisms responsible for the later rise of an acutely carbimazole suppressed uptake, the serum PB127I and TSH concentration was measured at weekly intervals. Whereas the PBI fell both in this and in another series of volunteers by the end of the fourth week of carbimazole treatment, no significant change in serum TSH concentration was measurable by radioimmunoassay. Nevertheless, the hypothesis is put forward that a biologically significant increase in serum TSH concentration had occurred, which was not detectable by the radioimmunoassay techniques available at the present time. There are indeed reasons to suspect that biologically important variations in TSH secretion may occur which fail to bring the serum TSH concentration beyond the normal range defined by radioimmunoassay.