Thyroid gland

The thyroid is a butterfly-shaped, bilobed gland lying just below the larynx on either side of the upper trachea, its two lateral lobes joined by a median bridge of tissue called the isthmus. It is the largest of the exclusive endocrine glands. Each lobe is made of many lobules, and each lobule is made of spherical follicles (acini), lined by cuboidal or squamous glandular epithelial cells, whose central lumen is filled with a thick glycoprotein material called colloid (rich in the protein thyroglobulin). Scattered between the follicles are separate parafollicular ('C') cells.

The follicular cells are responsible for making the two 'major metabolic hormones' of the body: tri-iodothyronine (T3) and thyroxine or tetra-iodothyronine (T4). Iodine is an essential raw material for both — the body needs roughly 1 mg of iodine per week to make normal amounts of thyroxine, which is why common table salt is fortified (iodised) at a ratio of about 1 part sodium iodide to every 100,000 parts sodium chloride. Their secretion is governed by a classic negative-feedback loop (Figure 11.4): hypothalamic TRH stimulates the anterior pituitary to release TSH, which in turn drives the thyroid to secrete T3/T4; once thyroxine levels rise enough in the blood, it feeds back on both the pituitary and the hypothalamus to shut TSH (and TRH) secretion back down. Thyroxine itself regulates the basal metabolic rate (BMR) and heat production, stimulates protein synthesis and growth, is essential for skeletal and nervous system development, helps maintain blood pressure, lowers serum cholesterol, and needs to be at the right level for normal gonadal function. (One important caveat: not every enlarged thyroid is iodine-related — sporadic goitre is a genetic condition and occurs independently of …