THYROIDAL 131I TURNOVER IN ADULT MALE RATS AT DIFFERENT TIMES OF THE SAME DAY

Abstract

ABSTRACT Male rats were fed a low iodine diet for 20 days. On the day of the experiment one group of animals was injected with 131I at 8 a.m., while another group received the tracer at 6 p. m. Subgroups of animals then were sacrificed at different intervals. The low iodine diet induced hypertrophy and a low iodine content in the thyroid, and decreased the plasma PBI levels. The values of thyroid radioiodine uptake, percentual distribution of 131I in thyroid hydrolysates, as well as the % plasma TCA precipitable 131I and PB131I, depended on whether the tracer was administered in the morning or in the afternoon, thus supporting the conclusion that the thyroidal 131I metabolism is highest during the period of darkness. Moreover, experiments on normal rats have demonstrated that the renal clearance and excretion of urinary 131I and 127I is higher during the active than during the resting period. It is suggested that the diurnal change in thyroid activity may be induced by a relative body and thyroid iodide deficiency through an increase in the urinary loss of iodine.