Progressive Age-Related Increase in Pituitary Thyrotropin Level and T/S Iodide Ratio in the Rat

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Abstract
Groups of normal Sprague-Dawley rats were killed at weekly intervals starting with the 1st postnatal day, and adenohypophysial TSH levels were determined by bio-assay. The TSH concentration increased progressively from 13 mU/mg in the newborn rats to over 100 mU/mg at 56 days. The T/S iodide ratio was significantly lower in 21-day-old rats than in 56-day-old rats, even though the sensitivity of the thyroid gland to exogenous TSH was slightly greater in the younger animals. It was concluded that the pituitary-thyroid system is not completely developed at birth, and that the eventual elevation in thyroid activity in older rats results from increased production of TSH rather than from enhanced sensitivity of the thyroid gland to TSH.