The thyroid is an endocrine---hormone-producing---gland located in the neck. Thyroid hormones act on every cell in the body. They control metabolism, or how the body uses energy from food and how it makes new proteins.
The most abundant hormone is thyroxine (T4), but 80 percent of it undergoes transformation into the almost four times more potent tri-iodothyronine (T3). The final hormone is calcitonin, which guards against bone calcium loss.
Function
Both thyroxin and tri-iodothyronine are important for growth and brain development, for regulating heart rate and body temperature. Many of their actions are mediated by binding to their receptors in the nucleus, where they interact with DNA and regulate genes and their translation into proteins. In almost every tissue in the body, excluding the brain, spleen and testicles, thyroid hormones stimulate metabolism.
Iodine and the Thyroid
Thyroid hormones require iodine for their synthesis. Iodine is abundant in seafood and in fruits and vegetables grown in iodine-rich soil. Iodine deficiency can cause goiter, which is swelling of the thyroid gland and is often prevented by the use of iodized salt. Insufficient iodine during fetal development can cause cretinism in children, which is severe impairment of physical and mental development.
Hormone Production and Release
Synthesis and release of T3 and T4 is controlled by a "negative feedback loop": Low blood levels of thyroid hormones stimulate the pituitary gland, the main endocrine gland of the body, to release thyroid-stimulating hormone (TSH), which further stimulates the thyroid to produce its hormones. That raises the blood levels of T3 and T4, the pituitary senses that and stops releasing TSH. The hypothalamus area of the brain also controls the pituitary gland; its thyrotropin-releasing-hormone (TRH) stimulates the pituitary gland to release the TSH.
Hyperthyroidism
Hyperthyroidism, or "overactive thyroid," is a condition with an excessive production of thyroid hormones. Symptoms include difficulty concentrating, restlessness, frequent bowel movement, heat intolerance, increased appetite and sweating. The most common is Grave's disease, caused by abnormal immune system response.
Taking too much iodine or thyroxine, and inflammation of the thyroid can also cause hyperthyroidism, which is usually treated with anti-thyroid medications, radioactive iodine to destroy the thyroid, or surgery to remove it. After the surgery or radiation, patients must take thyroid hormone-pills for the rest of their life.
Hypothyroidism
Hypothyroidism is insufficient production of the thyroid hormones. Symptoms include fatigue, depression, constipation, weight gain, cold sensitivity, dry skin, puffy face, thin hair and fingernails. Hypothyroidism is easily treated by replacing the deficient hormone.
Hashimoto's thyroiditis is chronic swelling of the thyroid that leads to hypothyroidism. It is caused by abnormal immune system response. Decreased thyroid hormone production in newborns causes neonatal hypothyroidism. Untreated, it can lead to severe mental and growth retardation. According to the NIH, most states require a routine screening test to check all newborns for hypothyroidism.
Other conditions
Swelling of the thyroid is called thyroiditis, probably caused by bacterial, fungal or viral infection. A small swelling or lump in the thyroid is a common condition called thyroid nodule. More than 99 percent of thyroid nodules are not cancer, according to the thyroid expert Dr. James Norman. There are four types of thyroid cancer, and most of them are curable, writes Norman online on EndocrineWeb.
