The Influence of Thyrotropin and Growth Hormone on the Thyroid Gland in the Hereditary Dwarf Mouse: A Morphometric Study*

Snell-type dwarf mice were injected with TSH, GH, or both hormones together for 6 days. GH induced an increase in body weight but not in the weight of the thyroid gland itself; on the contrary, TSH caused an increase in the weight of the thyroid but no increase in body weight. After TSH injection, the relative volume of the thyroid parenchyme was enhanced by 45% compared to that in untreated dwarf mice, and the radius of the follicles and follicular lumina increased by 50% and 48%, respectively. The major effect of TSH was an increase in cellular volume (+93%), and the mean number of cells in the average follicle was doubled, without a reduction in the number of follicles. GH had almost the same effect as TSH on the relative volume of the parenchyme and caused the radius of follicles and of the follicular lumina to increase by 61% and 69%, respectively. However, GH did not influence cellular volume. Its primary effect was to stimulate cellular division (cells were increased about 5 times in the average follicle) and to reduce the number of follicles. The nucleo-cytoplasmic ratio increased with GH but decreased with TSH. T4 serum levels increased to a much lesser extent with GH than with TSH, while normal values were obtained with both hormones together. At a morphological level, the combined administration of TSH and GH produced the same qualitative effects as separate administration, inducing an increase in cell volume and number which was less than the sum of the effects of each hormone administered separately.