The thyroid gland itself consists of numerous functional units or follicles of varying size, separated by connective tissue (Fig. 15.7a). Each follicle is spherical and lined by epithelial cells surrounding a central colloid space. The base of each cell is adjacent to a capillary, and the apex consists of microvilli pointing into the colloid.
The parafollicular or C-cells of ultimobranchial body origin secrete calcitonin. These cells are present between the follicles and usually do not touch the follicle lumen.
