Sympathetic innervation and noradrenaline content of normal human thyroid tissue from fetal, young, and elderly subjects

In man, as well as in the mouse, there is morphologic and functional evidence for a direct, stimulatory influence of the sympathetic nervous system on the secretion of thyroid hormone by noradrenaline (NA), released from interfollicular adrenergic nerve terminals. In mice and rats, an age-related reduction of the sympathetic innervation of the thyoid has recently been observed. In the present study, possible age-related variations of the sympathetic innervation and the concentration of NA in human thyroid tissue were examined. Interfollicular adrenergic nerve terminals were studied by fluorescence histochemistry, and the tissue concentration of NA was measured by fluorometry. In apparently normal thyroid tissue, obtained from fetuses, young (20-45), and elderly (greater than 60) euthyroid people with thyroid cancer or hyperparathyroidism, the number of interfollicular adrenergic nerve terminals appeared to be reduced with increasing age, and the thyroid tissue concentration of NA was significantly lower in elderly than in young people. These findings may have functional importance.