Effect of Hypothyroidism and Thyroid Hormone Replacementin Vivoon Pituitary Cytoplasmic Concentrations of Thyrotropin-β and α-Subunit Messenger Ribonucleic Acids*

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Abstract
We have studied the influence of hypothyroidism and thyroid hormone replacement in vivo on rat pituitary cytoplasmic concentrations of TSH.beta. and .alpha.-subunit mRNA, measured by cytoplasmic dot hybridization, as well as circulating TSH and pituitary TSH content. Cytoplasmic concentrations of GH, PRL, and LH .BETA.-subunit mRNA were measured in parallel. Marked increases in serum TSH and TSH.beta. and .alpha. mRNA were found in hypothyroidism; these changes reversed 72 h after beginning T3 replacement. A more marked effect of hypothyroidism on TSH.beta. mRNA than on .alpha. mRNA was evident, suggesting differential control of regulation of the TSH genes. In contrast to a rapid fall in serum TSH after T3 administration, an increase in TSH.beta. and .alpha. mRNA was evident at 1 and 6 h, followed by a later fall in TSH mRNA to euthyroid values. Pituitary cytoplasmic concentrations of GH and PRL mRNA were reduced in hypothyroidism and increased after administration of T3; LH.beta. mRNA was unaffected by hypothyroidism or T3 replacement. The differential effects of thyroid status on TSH, GH, PRL, and LH mRNA indicate specificity of regulation of these anterior pituitary hormone genes.