Capillary endothelial cell multiplication in adipose tissue pads on the thyroid during the feeding of thiouracil

It has been reported that blood capillaries in adipose tissue pads on the upper and lower poles of the thyroid gland enlarge when Fischer rats are fed thiouracil (TU) in a low iodine diet. To test whether the enlargement is accompanied by proliferation of the endothelial cells, [3H]thymidine was injected into rats fed the TU-containing diet, and labeling of the endothelial cells was studied by autoradiography. Nuclear labeling of the capillary endothelial cells was observed in the mixed brown and white adipose tissue (BAT and WAT, respectively) pads on the thyroid. After a single pulse of [3H]thymidine, 10% of the nuclei were labeled at 10 days (the peak labeling), and labeling decreased thereafter. To test whether the adipose tissue was stimulated because of the poor nutritional quality of the low iodine diet, Purina Laboratory Chow (a nutritionally adequate diet) was tested and produced the same result. To test whether TU had a direct effect, 5 micrograms T4/100 g BW were given daily; there was then no response to the TU, suggesting that the effect was due to an elevated circulating concentration of TSH. The effect was generally restricted to the adipose tissue pads on the thyroid. There was no response in interscapular BAT, epididymal WAT, or sc WAT. However, there was a response in small clusters of adipocytes embedded in inguinal sc WAT. The results are consistent with the idea that the effects are directly or indirectly due to elevated circulating TSH levels.