Abstract
ABSTRACT In order to study the mechanism of the paradoxical potentiating effect of small doses of thyroid hormone on propylthiouracil goiter growth in rats, serum and pituitary thyrotrophin (TSH) titers were measured after small and larger doses of triiodothyronine (T3) in thyroidectomized rats. A small dose of T3, 4 μg daily, sufficient to maintain normal oxygen consumption, caused elevated pituitary and serum TSH titers when compared to controls. Reduced titers of TSH resulted from a larger dose of T3, 16 μg daily. It is concluded that the goitrogenic action of small doses of thyroid hormone is not limited to an interaction peculiar to propylthiouracil administration but may be the result of direct stimulation of increased synthesis and secretion of TSH by the pituitary gland and is seen even in the absence of thyroid tissue or propylthiouracil. It is hypothesized that small amounts of thyroid hormone are necessary for optimal or maximal synthesis and secretion of TSH. These effects may be mediated directly at the pituitary gland or at a suprapituitary level. It was also found that stalk-median eminence TSH titers were significantly increased in the thyroidectomized rats that received a large dose of T3, as compared to those that received no T3, i. e. the effects were the opposite of those seen in the pituitary gland.
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