VANDERLAAN and Caplan (1954) and Halmi (1954b) have shown that the response of the thyroidal iodide concentrating mechanism of hypophysectomized rats to exogenous thyrotropin (TSH) is influenced by the iodine content of the diet. The same dose of TSH was found to be more effective in raising the thyroid: serum radioiodide gradient when iodine intake was low than when when it was high. A priori this phenomenon is amenable to three interpretation: a) the stable iodide of the diet, being the substrate of the concentrating mechanism, could diminish the gradient of the subsequently administered radioiodide by overloading the iodide concentrating mechanism, whose capacity is limited; b) prolonged elevation of serum iodide levels per se may actually depress the iodide concentrating mechanism and/or inhibit its responsiveness to TSH, or c) a rise in the level of serum iodide may exert such an influence indirectly by enhancing the concentration of organic iodinecontaining compounds in the thyroid.
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Halmi et al. (1955) studied this question.