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THYROID HORMONE SECRETION RATE IN THE LACTATING RAT

Summary.: Thyroid secretion rate (TSR) was determined in lactating and non-lactating rats by the thyroxine substitution and `direct output' methods. In lactating rats, apparent TSR as determined by the substitution method was significantly greater than that of controls when dietary iodine was deficient. By the direct output method, TSR was significantly reduced even when dietary iodine was adequate. It is concluded that the substitution method measures the physiological demand for thyroid hormone, which is greater during lactation, but represents true TSR only if iodine supplies are adequate. Results obtained by the direct output method show the actual TSR to be significantly reduced both during lactation and when dietary iodine is limited. Mean thyroid acinar cell height significantly increased as thyroidal iodine was depleted. It is concluded that, during lactation, TSH was elevated but iodine was insufficient to support the demand for thyroid hormone. The low serum T⁴ level observed in lactating rats, as determined by a competitive binding method, supports these conclusions.

Analysis of the methods of computation of TSR by direct output showed that (1) as thyroidal iodine supplies were depleted, thyroidal iodide accumulation became more efficient, (2) the iodine-deficient thyroid recycled a greater percentage of the available iodine, (3) thyroidal ¹³¹I uptake was reduced in lactating rats, (4) total thyroidal iodine was inversely correlated to its release rate (K₄) from the thyroid, and (5) thyroidal iodine, and not K₄, was the major factor influencing TSR under these experimental conditions.