Alterations of Radioimmunoassayable Growth Hormone and Prolactin during Hypothyroidism

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Abstract
Induction of hypothyroidism by treatment of rats with propylthiouracil (PTU) or radioactive iodine reduced pituitary and plasma growth hormone concentrations. Radioimmunoassayable pituitary and plasma growth hormone was diminished after three weeks of PTU ingestion. By four weeks, plasma growth hormone had fallen to barely detectable levels in most animals. Pituitary growth hormone concentration reached the lowest value by 40 days and remained depressed throughout 98 days of PTU ingestion. After treatment for 21 days with PTU, rats given tap water had progressively increasing pituitary growth hormone concentrations reaching control levels within 19 days. Rats given PTU and thyroxine injections to prevent hypothyroidism had no appreciable change in pituitary or plasma growth hormone concentrations. Administration of cortisol to chronically hypothyroid animals failed to increase pituitary growth hormone content. When rats bearing the growth hormone— and prolactin— producing tumor MtTWl5 were rendered hypothyroid, plasma and tumor growth hormone contents were substantially reduced. In contrast to the dramatic effects of hypothyroidism on plasma growth hormone concentration, plasma prolactin concentration was altered little by thyroxine deficiency. Although the mean plasma prolactin concentration in hypothyroid male rats was higher than the mean values found in euthyroid animals, the differences were not statistically significant. Pituitary prolactin content was reduced by about 50% in hypothyroid male and female rats indicating a modest reduction in the amount of prolactin stored in the lactotrope. The ability of the transplantable tumor MtTWl5 to produce prolactin was not diminished by the hypothyroid state. In fact, there were increased tumor and plasma prolactin concentrations in MtTWlSbearing hypothyroid rats. In these same animals the pituitary prolactin content was decreased by PTU to levels below those found in euthyroid tumor—bearing rats. These studies suggest that hypothyroidism selectively diminished growth hormone production from the somatotrope. No such direct effect of thyroxine deficiency on prolactin production from the lactotrope was found. The lowered pituitary prolactin storage seen in hypothyroid rats could be the result of an increase in prolactin inhibiting factor activity in these animals. The elevated plasma prolactin found in hypothyroid tumorbearing rats indicates that thyroxine deficiency did not decrease synthesis and secretion of prolactin. In fact, hypothyroidism lead to an apparent increase in prolactin production by lactotropes removed from hvpothalamic control. (Endocrinology92: 487, 1973)

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