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Studies in dogs have indicated that after the injection of thyrotropin (TSH) there is an enhanced release into thyroid venous blood of inorganic iodine in addition to the expected secretion of hormonal iodine (1, 2). Plasma iodide that has been newly accumulated by the thyroid iodide "trap" and still exists in the gland in inorganic form (transported iodide) does not appear to be the major source of the iodide released after TSH stimulation. The published data [reviewed in (3) ] suggest that the iodide so released is produced within the gland and is derived from deiodination of glandular organic compounds.However, a release of transported iodide into thyroid venous blood after TSH administration also has been demonstrated by studies in which organic binding of iodide was blocked (4, 5). The mechanism for the release of transported iodide and its relationship to the generation and release of iodide produced within the gland from organic precursors remain unresolved. It has been suggested (5) that the effect of TSH upon the release of transported iodide may be a consequence of the formation of iodide from glandular organic stores.The purpose of this report is to present data that provide more precise characterization of the effect of TSH upon glandular iodide release and of the * Submitted for publication October 22, 1965; accepted January 25, 1966. Supported by research grants A-2585 and A-5589 from the National Institute of Arthritis and Metabolic Diseases.Presented in part at the Forty- relationship of iodide derived from intraglandular organic stores to that newly accumulated from the circulation. Experiments were performed in which these two sources of glandular iodide were individually labeled in the same animal with different radioiodine isotopes whose flux between thyroid and circulation was estimated from differences between the radioiodide concentrations of arterial and thyroid venous plasma. Related studies on the acute effect of TSH on unidirectional (blood to thyroid) radioiodide clearance and on the sensitivity of iodide release to exogenous TSH stimulation are also reported. MethodsPentobarbital-anesthetized male mongrel dogs, weighing 30 to 50 pounds, were used in all experiments. The inferior thyroid vein of one lobe was cannulated, and continuous serial samples of thyroid venous blood were obtained by methods previously reported (1). The duration of each sampling period was usually 4 to 6 minutes; this was reduced to 2 to 3 minutes in the clearance experiments. Femoral arterial blood was drawn at the midpoint of each venous sample. Intravenous injections were given through a femoral vein cannula.Radioactivity measurements were made with a scintillation well counter and spectrometer. All samples were counted at a final volume of 1.0 ml. Separate 'I and 'I standards were made from suitable dilutions of the same isotope solutions administered to the animals. The activity present at each of two different energy levels was determined for each plasma sample and isotope standard with spectrometer wi...

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