The thyroid gland is one of the larger endocrine (ductless glands secreting hormone directly into the bloodstream) glands in the body. It is a butterfly or horseshoe shaped gland that lies in front of the trachea (windpipe) just below the thyroid cartilage (Adam’s apple). It consists of two lobes on either side of the trachea connected by an isthmus of glandular tissue crossing over the trachea. The recurrent laryngeal nerves that enter the muscles controlling movement of the vocal cords run behind the gland adjacent to the trachea. There are four, paired, tiny endocrine glands called parathyroid glands that make another hormone called parathormone. This hormone precisely maintains the proper balance of calcium in the bloodstream by regulating calcium absorption from food, mineral content in the bones and elimination by the kidneys. The parathyroid glands are located next to and usually behind the thyroid gland. These adjacent structures are of clinical importance since surgical removal of the thyroid gland places both of the nerves at risk of injury possibly resulting in permanent hoarseness due to a weak or immobile vocal cord. Inadvertent removal of the parathyroid glands in the course of thyroidectomy may result in temporary or rarely, permanent hypocalcaemia (low blood calcium levels). As noted earlier, the thyroid, as with all other endocrine glands, secretes hormones into the blood directly, rather than through a duct. The thyroid gland secretes three hormones. Two of these hormones; thyroxine or tetraiodothyronine (T4) and triiodothyronine (T3), control how quickly the body uses energy (metabolism) along with the growth and function of many body systems. Most of the circulating thyroid hormone is in the form of T4 and gets converted to T3 after entering individual cells as the more active form. The third hormone is calcitonin which is secreted by a special type of cell in the gland known as the parafollicular C-cell. Calcitonin has the opposite effect of parathyroid hormone resulting in the lowering of serum calcium levels. The amount and rate of secretion of the two thyroid hormones, T3 and T4 is controlled by the pituitary, another endocrine gland, located at the base of the brain, which secretes thyroid stimulating hormone (TSH). The control of thyroid gland secretion by the pituitary gland is via a “negative feedback” loop functioning much like a thermostat. As the levels of T3 and T4 rise in the blood, the amount of TSH secreted by the pituitary is diminished. Conversely, as the levels of the two hormones in the bloodstream fall the pituitary releases more TSH resulting in an increase of T3 and T4 secretion.

Impairment of thyroid gland function and the attendant decrease in thyroid hormone levels in the body causes profound changes in many body systems yielding the clinical syndrome of hypothyroidism (too little thyroid hormone). Hypothyroidism may be brought about by disease of the thyroid gland itself or by removal of the gland for malignant tumors or enlargements (goiters) causing pressure on surrounding structures resulting in difficulty swallowing and breathing. Hypothyroidism may also result from treatment of hyperthyroidism caused by Graves’ disease with radioactive iodine, external beam radiation to treat other kinds of cancers in the head and neck, and insufficient dietary iodine. The failure of the gland to function due to problems with the gland itself is termed primary hypothyroidism. Conditions that interfere with the pituitary gland regulating thyroid secretion due to failure of TSH secretion are termed secondary hypothyroidism. Finally, disruption of the signals from the area of the brain (hypothalamus) signaling the pituitary to secrete TSH is known as tertiary hypothyroidism. Congenital hypothyroidism is a form of primary hypothyroidism where the infant has failed to develop the thyroid gland during fetal development. The older, no longer used term for this condition is “cretinism”. If unrecognized and untreated, these unfortunate children develop severe mental retardation, growth problems and heart problems. In adults who become hypothyroid, symptoms and signs may include: weight gain, fluid retention, muscle cramps, constipation, depression, cold intolerance, slowing of the heart rate (bradycardia), enlargement of the thyroid gland (goiter) and fatigue. Hypothyroidism rarely reaches the point where it is clinically apparent, as tests for thyroid function are routinely included in blood tests for annual checkups. Even prior to clinical symptoms, the diagnosis is based on blood tests indicating below normal levels of thyroid hormone.