A TRANSIENT RISE OF HORMONE SECRETION: A RESPONSE OF THE STIMULATED RAT THYROID GLAND TO SMALL INCREMENTS OF IODIDE SUPPLY

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Abstract
Small doses of iodide (2 times 3.2 .mu.g at 12 h interval), below those capable of inducing Wolff-Chaikoff effect, were injected into rats kept on a moderately low I diet. By means of a 125I equilibration technique and by direct measurement of cold thyroxine, it was demonstrated that the level of circulating PB125I (representing iodothyronines as confirmed by column chromatography) increased by a mean of 40% within 24 h following the 1st iodide injection. The serum TSH (thyrotropin) concentration (measured by radioimmunoassy) was simultaneously depressed. In stimulated thyroid glands, a biologically significant fraction of an iodide load escapes autoregulatory control of iodothyronine synthesis. A small, transient increase of hormone release is likely to represent the physiological response of a normal gland to a sudden supplement of iodide supply. The ensuing depression of TSH secretion may be necessary for final adjustment of thyroid function. It is considered to be the last step in a cascade of mechanisms whose interaction keeps the thyroidal hormone output within narrow limits in the face of a fluctuating iodide supply. Failure of 1 or more of these mechanisms in humans is discussed.