What Are the Functions of the Thyroid?

Thyroid Location
The thyroid is one of the major glands of the endocrine system located in the neck, below the larynx or voice box. The structure of the thyroid is similar to the shape of a butterfly with two lobes, one on each side of the trachea or windpipe connected by a band of tissue called the thyroid isthmus. The primary function of the thyroid is to stimulate and maintain metabolic processes through the production and release of hormones. Three hormones produced by the thyroid are thyroxine (T4), triiodothyronine (T3) and calcitonin. Dysfunction of the thyroid can result in medical conditions known as hyperthyroidism and hypothyroidism.

T3 and T4 Hormone Production

Thyroxine Molecular Structure
The thyroid takes iodine from the blood to produce two very similar amine hormones that contain iodine. Thyroxine is referred to as T4 because it contains four iodine atoms. Triiodothyronine is referred to as T3 because it contains three iodine atoms. These hormones are released into the blood stream to increase the rate of oxygen consumption and cellular metabolism to produce heat and energy for biological processes. Thyroid hormones stimulate growth and maturation during child development as well as increased mental activity. T3 and T4 also play a role in the maintenance of normal blood pressure, heart rate, muscle tone, digestion and reproductive functions.

Calcitonin is produced by the thyroid in response to high calcium levels in the blood. The primary function of calcitonin is the metabolism and maintenance of calcium through induction of kidney secretion and bone storage. When released, calcitonin induces the kidneys to increase the amount of calcium released in the urine. Calcitonin also raises the amount of calcium stored in the bones.

An overactive thyroid, known as hyperthyroidism, is when there is an excess production of thyroid hormones. This condition causes an increase of the body's metabolic processes. Common symptoms are excessive weight loss, anxiety, fatigue, hair loss, weakness, hyperactivity and irritability.

Goiter of a young woman.
An underactive thyroid, known as hypothyroidism, is when there is a deficiency in thyroid hormone production. This condition causes a decrease in the body's metabolic processes. Common symptoms are paleness, brittle fingernails and hair, muscle cramps and joint pain, fatigue and goiter-swelling of the thyroid gland. A form of hypothyroidism found in infants is known as cretinism.