The endocrine system is composed of a number of glands, including the thyroid. The thyroid has a shape similar to that of a butterfly and is located slightly below the larynx. It consists of two lobes, one to either side of the windpipe. The lobes are connected by a small piece of tissue referred to as the isthmus.
The thyroid gland is composed of specialized sacs referred to as follicles. The thyroid follicles are globular (spherical) in shape. The cells of the follicles have one function which is to produce thyroglobulin, a precursor to true thyroid hormones. When the thyroid is not secreting hormones, the sacs are filled with thyroglobulin. When the body requires thyroid hormones, the thyroglobulin is broken down to produce the hormones which are secreted into the bloodstream.
The thyroid gland is deceptively simple in that its sole function is produce two hormones: thyroxine and triiodothyronine, called T4 and T3 respectively. T4 and T3 are modified versions of an amino acid called tyrosine. Thyroid hormones carry iodine molecules, four molecules in T4 and three molecules in T3. T3 is the more potent of the two hormones and in many instances T4 is converted into T3 in cells. When these hormones are secreted, they usually bound to a protein called TBG (thyroxine binding globulin).
It is fair to say that virtually every cell in the human body is affected by thyroid hormones. When thyroid hormones enter cells, their primary purpose is to encourage protein production. They also stimulate mitochondria, which are the energy production centers of cells. The thyroid hormones also control how much energy is used for body functions or released as heat. In a general sense, thyroid hormones are responsible for virtually all metabolic activity in the body from appetite to nerve function and cardiac (heart) function.
Any hormone that exerts that much control over the body must, itself, be regulated in some fashion. The primary way in which the body regulates thyroid hormone production is through a process called a negative feedback loop that affects the hypothalamic-pituitary-target organ axis. As the amount of thyroid hormone in the body increases, it inhibits the hypothalamus and pituitary glands from releasing hormones that would make the thyroid release more hormones. As the amount of thyroid hormones in the body decreases, it allows the hypothalamus and pituitary gland to release the hormones that cause the thyroid to secrete T3 and T4.
The two most common problems that affect the function of the thyroid are hyperthyroidism and hypothyroidism. Hyperthyroidism occurs when the thyroid is overproducing the thyroid hormones. It can lead to a variety of symptoms, ranging from muscle weakness and heart palpitations to fatigue and insomnia. Hypothyroidism occurs when the thyroid is under-producing thyroid hormones. Common symptoms of this condition include irregular menstruation, fatigue, weight gain, and intolerance for the cold.