The Thyroid Gland
The thyroid produces two main hormones. Thyroxine (T4) and its more active counterpart triidothyronine (T3) regulate body metabolism, while calcitonin decreases the rate of bone breakdown.
Anatomy
The thyroid is an important endocrine gland located below the larynx, in front of the trachea. The two bulky lobes of the thyroid wrap part of the way around the trachea, connected by the isthmus. The four small parathyroid glands are embedded in the back of the thyroid, and the recurrent paryngeal nerves run closely alongside the thyroid.
The adult thyroid is normally 15-25 gm, and is divided into 20-40 evenly dispersed follicles by thin fibrous septae. Follicles range from 50-500 µm in size and are comprised of cuboidal or low columnar epithelium. Follicles surround colloid.
Thyroid Regulation
Thyroid-stimulating Hormone (TSH) is released from the anterior pituitary, in response to signals from the hypothalamus via TSH-releasing hormone (TRH). Somatostatin and dopamine, released from the hypothalamus, can inhibit TSH release from the pituitary.
During puberty, pregnancy, and physiologic stress, the thyroid increases in size through transient hyperplasia and becomes more active. Follicular cells become taller and more columnar, sometimes forming small inward folds or papillae.
Thyroid and Disease
According to the Thyroid Foundation of Canada, one in three Canadians has a thyroid disorder. The two main types of disease are hypothyroidism and hyperthyroidism, each with various causes and subtypes. Thyroid cancer is one of the most serious thyroid diseases.