Sympathetic innervation of the thyroid: variation with species and with age.

Fluorescence histochemistry was used to study the sympathetic innervation of the thyroids from adult individuals of six different species; mouse, rat, hamster, dog, sheep, and pig. In addition, thyroids from very young rats and from very old mice were examined. Generally, thyroidal sympathetic, adrenergic nerve terminals were found not only as a network around vessels, but also as single terminals between, and sometimes around, follicles. Interfollicular terminals were numerous in thethyroids of adult mice, sheep and hamsters, but they were few in the thyroids of adult rats and dogs, and even fewer in the porcine thyroid. In contrast to the findings in thyroids from adult mice and rats, several interfollicular terminals were found in thyroids from very young rats while very few such terminals were detected in the thyroids from very old mice. The observations suggest that there is a pronounced interspecies variation in the number of thyroidal interfollicular sympathetic nerve terminals and that, at least in the rat and the mouse, there is also a variation with age. Since, in mice, sympathetic activation appears to induce thyroid hormone secretion by a direct action of norepinephrine released from intrathyroidal sympathetic fibers, the recorded variations are presumed to have functional importance.