Sympathetic Innervation of the Mouse Thyroid and Its Significance in Thyroid Hormone Secretion

Studies were performed in mice to investigate the sympathetic innervation of the thyroid and to evaluate its role in the regulation of thyroid hormone secretion. Fluorescence histochemistry and electron microscopic autoradiography were used to study the distribution of adrenergic fibers and terminals in the thyroid and their relation to follicle cells and vessels. Bilateral surgical sympathectomy was performed by extirpation of the superior cervical ganglia, and pharmacological sympathectomy by iv injections of 6-hydroxydopamine. The secretion of thyroid hormone was estimated by daily measurements of the blood radioiodine (BRI) levels. Sympathetic, adrenergic nerve fibers were numerous in the mouse thyroid, and were mainly found in 2 locations: in relation to arterioles, and in interfollicular spaces. The terminals of the latter group had a very close relation to follicles and often were in direct contact with them, only the follicular basement membrane separating the adrenergic terminal and the follicle cell. In addition, interfollicular terminals often had a close relation to capillaries. It is concluded that in mice there is a morphological basis for a direct, nonvascular influence of the sympathetic nervous system on thyroid follicle cells. In addition, sympathetic stimuli may influence thyroid blood flow by effects on both arterioles and capillaries. Both pharmacological and surgical sympathectomy elicited a reduction in BRI levels that persisted for 1 or 2 days, presumably as a reflection of decreased thyroid hormone secretion. In view of this finding, and since a previous study has shown that unilateral sympathetic stimulation causes a large increase in BRI levels, preceded by formation of colloid droplets in the portions of the thyroid supplied by the stimulated nerve, it is suggested that the sympathetic innervation of the thyroid serves as a means for effecting prompt, short-term alterations in the rate of thyroid hormone secretion. (Endocrinology94: 959, 1974)