Increases in thyroid gland blood flow after hemithyroidectomy in the rat

After subtotal thyroidectomy, the thyroid gland remnant undergoes compensatory alterations in function and morphology. Under the trophic stimulation of elevated plasma TSH concentrations, the thyroid remnant responds with an increase in hormone synthesis and secretion and, in addition, increases in mass. We have examined the alterations in thyroid blood flow which accompany increased secretion and growth after hemithyroidectomy (HTX) in male Sprague-Dawley rats (200-220 g). At various times after surgical HTX (1, 2, and 3 weeks), blood samples for the determination of plasma hormone concentrations were obtained and tissue blood flows were determined using 15 +/- 5 microns diameter 141Ce-labeled microspheres in a modification of the reference sample microsphere technique. The microspheres were injected directly into the left cardiac ventricle via a 23-gauge needle passed through the chest wall while a reference blood sample was collected. After the animals were killed, tissues were cleaned and weighed, then tissue and reference blood sample radioactivities were determined. In addition, thyroidal immunoreactive vasoactive intestinal peptide was measured after acetic acid (0.67 N) extraction. After HTX, plasma TSH concentrations were significantly elevated. The plasma concentrations of T4 and T3 fell, but by less than the expected 50%. The mass of the remaining thyroid lobe increased progressively over the 3 weeks post thyroidectomy, reaching approximately 70% of the total thyroid gland weight of sham-operated controls. Thyroid blood flow per gram of tissue was significantly elevated at all times post HTX. HTX did not induce any alterations in thyroidal immunoreactive vasoactive intestinal peptide concentration. Thus, after HTX, the well documented compensatory alterations in thyroid remnant growth and secretion were accompanied by a prompt and striking increase in thyroid blood flow.