The thyroid is a small endocrine gland inside the neck, located in front of the breathing airway (trachea) and below the Adam's apple. It produces two thyroid hormones, tri-iodothyronine (T3) and thyroxine (T4), which circulate in the bloodstream to all tissues of the body.
Thyroid hormones act to control metabolism, which is the body's ability to break down food and release energy in the process.
How well the thyroid works is determined by another gland called the pituitary. The pituitary gland is located at the base of the brain and produces thyroid-stimulating hormone (TSH). TSH circulates via the bloodstream to the thyroid gland where it activates the thyroid to produce more thyroid hormones.
Hyperthyroidism: an overactive thyroid gland. Too much thyroid hormone production speeds up the body's metabolism. Women get this condition more often than men; it occurs in about 1% of women. One of the most frequent forms of hyperthyroidism is known as Graves' disease (named after Dr. Robert Graves). This condition can run in families, although the exact nature of the genetic abnormality is unknown.
Hypothyroidism: an underactive thyroid gland. Several causes for this condition exist, most of which affect the thyroid gland directly, impairing its ability to make enough hormones. More rarely, there are conditions in the brain (for example, pituitary tumors) that cause the pituitary gland to fail to produce thyroid-stimulating hormone (TSH) and stimulate the thyroid enough to make hormones.Whether the problem is with the thyroid gland or the pituitary gland, the result is that the thyroid is not producing enough hormones, so most major body functions, including physical and mental processes, slow down. The body consumes less oxygen and produces less body heat.