Light micrograph of the human thyroid gland, showing the functional units of thyroid follicles, each surrounded by a single layer of cuboidal epithelial cells. The follicles vary in size according to their states of synthetic or secretory activity; some contain stored hormones in the form of thyroid colloid, which appears red. Two types of hormones are produced: 1) The iodine- containing hormones tri-iodothyronine (T3) and thyroxine (T4): T4 is converted to T3 in circulation - T3 regulates basal metabolic growth. 2) calcitonin, which regulates calcium levels in the blood. Magnification: x100 at 35mm size.