Three-dimensional imaging of blood vessels in thyroids from normal and levothyroxine sodium-treated rats.

Three-dimensional images of the microvascular architecture in the thyroid glands of levothyroxine sodium-treated (for 4 weeks) rats were examined by scanning electron microscopy of corrosion casts made by the injection of methacrylate resin. In the experimental animals, each follicle was surrounded by a clearly defined basket-like capillary network generally independent of the adjacent one in the same manner as in normal animals. However, the capillaries were markedly poor in distribution, conspicuously small in diameter, and their anastomoses were clearly decreased as compared with the normal state. The diameters of the capillaries were 3-8 microns in these cases, while those in normal specimens were 5-15 microns. The capillary bed surrounding each follicle occupied about 25% of the follicular surface area, while that in normal cases occupied about 50%. In conclusion, it became clear that the distribution and morphology of the thyroid capillaries are markedly affected and changed by the functional state of the gland.