HypothyroidismHypothyroidism is a chronic disease that is part of the endocrine system. Hypothyroidism is due to low or lack of the thyroid hormones tri-iodothryonine (T3) and tetraiodothyronine (T4) produced by the thyroid gland. Lack of thyroid hormones could be due to a deficiency of iodine or thyroid-stimulating hormone (TSH) or an underactive thyroid gland (Sherwood, 2012). The thyroid hormones T3 and T4 are involved in almost every part of the body. It is ten times more prevalent among women than men, affecting approximately 4.6% of the American population (Lellis-Santos et al, 2011). In most cases, symptoms are cold intolerance, fatigue, bradycardia and weight gain. Depending on the cause of hypothyroidism, a goiter may develop, which is an enlargement of the thyroid gland. In most cases, taking a thyroid hormone supplement is an effective treatment. In a healthy individual, the thyroid gland produces the thyroid hormones T3 and T4 through a series of steps. Iodide is transported into the thyroid follicular cell from the bloodstream by symporters. Iodide is transported into the cell against its concentration gradient, using the Na+ concentration gradient as an energy source. Follicular cell iodide is activated by oxidation by thyroperoxidase (TPO). Active iodide is passively transported into the colloid via an iodide channel located on the luminal side of the follicular cell down its concentration gradient. Thyroglobulin (Tg), another precursor of thyroid hormone, enters the colloid by exocytosis from the Golgi complex of the follicular cell. Thyroglobulin (Tg) binds to iodide using thyroperoxidase to form monoiodotyrosine (MIT). If another iodide is added to the resident tyrosine of Tg, di-iodotyrosine is formed (DIT...... middle of paper ......wn. Thus, they are stored instead. As a result , people with low levels of thyroid hormones experience weight gain Also due to a decrease in fat metabolism, there is a decrease in heat production, thus leading to cold intolerance. effect of thyroid hormone deficiency is that bradycardia plays an important role in regulating cardiac activity T3 binds to the thyroid hormone receptor alpha in the cell nucleus. The thyroid hormone response element, which activates the myosin production gene (CITE Hormone ACTIOn) is a cytoskeletal protein important for contractility of the heart. With low levels of thyroid hormones, the chain gene. Heavy myosin beta is not expressed, leading to decreased contractility of the heart. So, people with hypothyroidism tend to have a slower heart rate or bradycardia..