Introduction to the Thyroid gland and Thyroid hormone
The thyroid gland is located in the neck just below the larynx. It is highly vascularized so that secreted hormones can be quickly moved away into the blood flow. This image shows a low powered microscopic view of the thyroid gland tissue. The thyroid gland consists of follicles which store large amounts of TH within a larger molecule called thyroglobulin. This protein is synthesized by the surrounding follicle cells and secreted into the follicles. We need iodine in our daily diet, but its only use is for the incorporation into TH. T3 (triiodothyronine) consists of 2 tyrosine amino acids and 3 iodines. T4 (tetraiodothyronine) consists of 2 tyrosines and 4 iodines. Both forms are secreted, although much the T4 is converted later into the more active T3 form. When TSH stimulates the release of TH, Thyroglobulin is taken into the follicle cells by endocytosis and broken down into T3 and T4, which then be released. TRH (Thyrotropin releasing hormone) from the hypothalamus stimulates the release of TSH from the pituitary gland. TSH (Thyroid stimulating hormone) stimulates both the synthesis and secretion of T3 and T4. Under normal conditions the level of TH stays relatively constant and this is regulated by negative feedback. TH inhibits the secretion of TRH and TSH. However, secretion can be modified, hypothermia and other stressors can increase TRH secretion whereas prolonged fasting will decreases TRH secretion. The role of thyroid hormone is to maintain resting metabolic rate and it affects most cells throughout the body. The hormone is lipophilic and can therefore cross the cell membrane. T3 regulates cell activity by controlling the proteins involved in energy metabolism. It does this by binding to receptors in the nucleus which regulate the rate of transcription of various genes. Normal metabolic rate depends on an adequate supply of TH. TH increases the production of ATP by increasing the metabolism of glucose and fats. This is achieved by boosting the number and activity of mitochondria and enhancing the production of glycolysis enzymes. TH also increases the use of ATP by stimulating the production of the Na/K ATPase pump. Metabolic reactions produce heat and this heat is important for maintaining normal body temperature. TH also has permissive effects, that is, it is required for the normal functioning of other pathways including sympathetic and growth hormone mediated pathways. In people with Hypothyroidism, the thyroid gland does not produce enough thyroid hormone. Hypothyroidism can be caused by a number of reasons including lack of iodine in diet, an autoimmune disease that attacks the thyroid gland (hashimotos disease) or inadequate TSH or TRH release from the pituitary gland. The most common form of Hyperthyroidism is Graves disease, an Autoimmune disease whereby an Ab is produced that mimics TSH.