Use of Technetium-99m in Studying Thyroid Physiology in the Mouse: Double Label Experiment with Technetium-99m and Iodine-1251

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Abstract
Mice were maintained for 72 hr. prior to the experiment at 30, 24 or 5C to induce hypoactive, normal or hyperactive thyroids, respectively. Pertechnetate is concentrated by the thyroid, probably be a transport mechanism similar to that for iodide, but is not bound or metabolized there. Carrier-free Tc99m in the form cf pertechnetate was used to determine thyroid/serum (T/S)3 ratios in the mouse thyroid gland not blocked by goitrogens. These T/S[TcO4-] ratios were similar to T/S[I~] or T/S[TcO4 "] ratios determined in goitrogen-blocked mouse thyroids. The T/S ratios of all groups of mice were inversely related to environmental temperature. One group of mice at each temperature receiving no goltrogen was injected with both TcO4 -99m and 125I- 1 hr prior to killing. Radioactivity of each of these radioisotopes was determined in plasma and throid glands by [UPSILON]-ray spectrometry in a well-type scintillation counter. In this double label experiment, 3 thyroidal responses were determined in each animal: an estimate of the "thyroidal iodide pump" (T/S[TcO4 -]) comparable to the T/S ratio of a blocked gland; an estimate oi the "thyroidal iodide pump" (T/S[I-]) in the unblocked gland; an estimate of thyroidal radioiodide uptake and binding. By combining all 3 thyroidal responses for each animal into one composite response metameter, more accurate discrimination of over-all thyroid status was made between groups of mice at each temperature.