The thyroid and parathyroid glands are situated in the neck, where they produce and secrete hormones into the bloodstream. There are two types of thyroid hormone, both of which help to control the rate of metabolism (the chemical reactions constantly occurring in the body). The four small parathyroid glands produce a hormone that controls calcium levels in the blood.
Thyroid disorders are common, but their onset is often gradual and they may not be detected or diagnosed for months or even years. Low levels of thyroid hormones at birth can prevent normal development of the brain and, for this reason, a blood test for thyroid hormone levels is one of the first tests to be performed on a newborn baby (see Blood spot screening tests). Over- and underactivity of the thyroid gland are the most common thyroid disorders and are discussed first. Swellings and growths are covered next, followed by disorders in which there is over- or underproduction of hormones by the parathyroid gland.
The last article describes multiple endocrine neoplasia, a group of rare inherited disorders in which tumours develop in several endocrine glands, including the thyroid and parathyroid.
For further information on the structure and function of the thyroid and parathyroid glands, see Hormones and Metabolism.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.