Effects of Small Doses of Thyroid Hormones on Thyroid Weight in Hypothyroid Rats

Placing rats soon after weaning on a low idoine diet and KClO4 resulted in hypothyroidism which was intense enough to produce effects similar to those following thyroid-ectomy on the plasma PBI, body growth and pituitary content of a protein associated with GH. Plasma insulin levels were also very low. Once growth stasis had occurred, l-T4 in very low doses (0.13−0.28 μg/100 g/day) or KI (1−11.5 μg I/day) was administered, while KClO4 treatment was continued until the animals were killed. These treatments were accompanied by an increase in the weight of the body and organs, the pituitary protein associated with GH, the plasma insulin levels, the plasma PBI, and sometimes the thyroidal 127I content. The weight of the thyroid gland increased significantly. The last of these effects was induced by exogenous insulin in the rats on low iodine diet + KClO4, although plasma TSH activity did not increase further. The results are discussed in relation to the hypothesis that a) the “goitrogenic” effects of small doses of thyroid hormones might be a consequence of partial restitution of the function of the pituitary and/or glands dependent on it, and b) full response of the thyroid to intense stimulation by TSH might require adequate levels of other hormones. (Endocrinology83: 41, 1968)