Thyroid Gland

The thyroid gland is the largest of the endocrine glands in the human body, a soft, highly vascular, bilobed structure lying in the front of the neck, its two lobes situated one on either side of the trachea just below the thyroid cartilage of the larynx, and joined across the midline, in front of the trachea, by a narrow band of tissue called the isthmus. Internally, the gland is made up of a very large number of small, roughly spherical structural units called thyroid follicles. Each follicle consists of a single continuous layer of cuboidal follicular epithelial cells surrounding a central, fluid-filled cavity containing colloid, a viscous material rich in the protein thyroglobulin, on which the thyroid's hormones are actually synthesised and stored before release.

The thyroid gland secretes two closely related hormones from its follicular cells: thyroxine (T4, so called because it carries four iodine atoms per molecule) and triiodothyronine (T3, carrying three iodine atoms). Because both hormones are built around iodinated derivatives of the amino acid tyrosine, an adequate dietary supply of iodine is essential for their synthesis; where dietary iodine is insufficient, thyroid hormone output falls, a deficiency taken up later in the chapter under simple (iodine-deficiency) goitre. T3 and T4 are synthesised within the follicular colloid, stored there bound to thyroglobulin, and released into the blood — under the stimulus of thyroid-stimulating hormone (TSH) from the anterior pituitary — as and when needed; T4 is secreted in the far larger quantity of the two, but much of it is converted to the more biologically active T3 within peripheral target tissues.

T3 and T4 together exert an unusually broad range of physiological effects, touching almost every tissue in the body. Their best-known role is the regulation of the basal metabolic rate (BMR) — the resting rate at which the body consumes oxygen and generates heat — by increasing the rate of cellular respiration in most tissues; a rise in circulating T3/T4 raises BMR, while a fall lowers it, which is why disorders of thyroid hormone level produce such wide-ranging effects on energy level, body temperature and weight. Thyroid hormones are also essential, in the correct amount, for normal physical growth and for the development of the nervous system, particularly during fetal life and early infancy, a dependence whose disruption underlies the disorder cretinism discussed later in the chapter. Beyond metabolism and growth, T3/T4 also promote red blood cell production, help maintain the balance of water and electrolytes in the body, and influence the normal functioning of the cardiovascular and reproductive systems. …