Anatomy
The thyroid gland is located anterior to the cricoid cartilage in the anterior neck, composed of two lobes connected by a branch of tissue known as the isthmus (Figure 47.1), which lies over the second and third tracheal rings. The thyroid is a very vascular organ; in conditions of severe hyperplasia, greater blood flow rates may become audible as a noise (bruit). Two pairs of parathyroid glands are located on the posterior surface of the thyroid gland.
Histology
The thyroid gland is composed of a large number of follicles (Figure 47.2). Each follicle is formed by a single layer of cuboidal epithelial cells (follicular cells), which secrete into the interior of the follicle; the lumen is filled with a proteinaceous, viscous material called colloid, which acts as a store for thyroid hormones (on the glycoprotein thyroglobulin). Scattered around the follicles are larger parafollicular or C cells that produce calcitonin. Within the parathyroid gland, parathyroid hormone (PTH) is secreted by chief cells, while the function of oxyphil cells remains unknown.
Parathyroid hormone and calcitonin
Parathyroid hormone
Secretion of PTH is stimulated by low plasma calcium levels. PTH acts to raise plasma calcium levels by stimulating calcium resorption of bone, stimulating calcium uptake from the gastrointestinal tract, enhancing renal excretion of phosphate. PTH is required for renal ‘activation’ of vitamin D into 1,25-dihydroxycholecalciferol, which acts to promote calcium reabsorption from the gut.
Calcitonin
Calcitonin is released in response to high plasma calcium levels. Calcitonin lowers plasma calcium by inhibiting bone resorption and inhibiting reabsorption in the kidney.
Thyroid hormones: thyroxine (T4) and tri-iodothyronine (T3)
Synthesis
Thyroid hormones are the only hormones that require an essential trace element, iodine, for the production of active hormone. The follicular cell actively accumulates iodide (I–) ions via the Na+-I– symporter (driven by the inward Na+ gradient), expressed on the basement membrane (this process is also called iodide trapping, Figure 47.3). At the apical (or luminal) membrane, I– is transported into the follicle lumen by a transporter called ‘pendrin’. I– is oxidised into an active intermediate by the enzyme thyroid peroxidase (or thyroperoxidase, TPO). TPO also catalyses the iodination of tyrosine residues (organification) on thyroglobulin (a glycoprotein rich in tyrosine residues), to produce mono-iodotyrosine (MIT) and di-iodotyrosine (DIT); and the coupling of two DIT molecules to form thyroxine (T4), or one MIT with one DIT to give tri-iodothyronine (T3) (Figure 47.4). The thyroid hormones are stored as part of the thyroglobulin molecule (colloid), with sufficient amount to supply the body with its normal requirements for 2–3 months.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
