A new method for the measurement of thyroidal iodine release in man

A new method is described for the detection of variations in the release rate of all iodinated products by the thyroid gland in man. (125)I was employed to endogenously label the thyroid gland, and a parenterally administered (131)I thyroxine dose was used to generate a (131)I reference source. Thyroidal iodine release activity was quantitatively assessed by the measurement of variations in (125)I/(131)I ratio values obtained in timed urine samples. Diurnal variation in thyroid release patterns was observed in euthyroid subjects which was promptly suppressed by exogenous triiodothyronine administration and was simulated by the intramuscular injection of 0.25-0.50 U of bovine thyrotropin. The zenith value occurred at 4:00 a.m. +/-3.4 hr (SD) and the nadir at 5:00 p.m. +/-3.6 hr. The absence of a diurnal pattern in patients with thyrotoxicosis and in secondary hypothyroidism indicated that this diurnal fluctuation was under thyrotropin (TSH) regulation. This new method also promises to be a useful tool for the study of the intrathyroidal recycling of iodide from the iodotyrosine pool and the detection of factors which may acutely alter thyroid function.