Thyroid Neuroendocrinology: Neural Regulation of Thyroid Hormone Secretion*

THE THYROID gland consists of two different endocrine systems, that of the follicular cells and that of the parafollicular cells (1). The follicular cells, which are closely packed cells surrounding a colloidal space, synthesize and secrete the thyroid hormones (T4 and T3) (2). The parafollicular cells (C cells) are found between these follicular cells, sometimes occurring as single cells, sometimes as a few cells together (3). These C cells, which may also be seen inside the follicular basal membrane (4), synthesize and secrete polypeptides. Thus, calcitonin (5, 6), somatostatin (7–10), calcitonin generelated peptide (CGRP) (11), and katacalcin (12), are all produced by the thyroidal C cells. Morphological studies of the thyroid have demonstrated structures other than the follicular and parafollicular cells. Thus, a vast number of blood vessels and fibrocytes form fibrous sheets dividing the gland into smaller units are seen within the gland (1). Furthermore, mast cells have been demonstrated to occur both in a parafollicular and in a perivascular location within the thyroid gland (13–15); interestingly, these mast cells are known to secrete histamine and serotonin upon stimulation by TSH (16, 17).