Relationship of age to pituitary TSH levels and PTU-induced goitrogenesis in the rat

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Abstract
Thyroid weights, adenohypophysial and serum thyroid-stimulating hormone (TSH) levels, and thyroid: serum (T/S) radioiodide concentration ratios were determined in control rats and in PTU-treated littermates of various ages. Adenohypophysial TSH levels were low in the immature control rats and increased progressively with age. Some of the weanlings showed little or no thyroid hypertrophy after treatment with PTU, although others displayed the standard goitrous response. Such variability was not observed in adults. In the goiter-resistant rats, adenohypophysial TSH levels were equivalent to the control values, and serum TSH was below the bio-assay threshold concentration of 0.5 mU/ml, indicating lack of significant release of adenohypophysial TSH. In the goitrous rats, adenohypophysial TSH declined and serum TSH increased, indicating that TSH was discharged from the adenohypophysis. Increases in both T/S ratio and thyroid weight occurred in adult rats treated with PTU for 1 week; but in comparably treated immature rats there was a dissociation of these responses in that the T/S ratio increased without a concomitant elevation in thyroid weight. The low endogenous level of adenohypophysial TSH in the immature rats and failure of TSH release in response to PTU tentatively are considered to result from the absence of physiological maturation of the hypothalamic-pituitary pathway involving control of TSH production and/or release.