Hypothyroidism Hypothyroidism is a chronic disease that is part of the endocrine system. Hypothyroidism is due to low or lack of thyroid hormones tri-iodotryonine (T3) and tetraiodothyronine (T4) produced by the thyroid gland. Lack of thyroid hormones could be due to an iodine or thyroid-stimulating hormone (TSH) deficiency or an underactive thyroid gland (Sherwood, 2012). Thyroid hormones T3 and T4 are involved in almost every part of the body. It is ten times more prevalent in women than in men and affects approximately 4.6% of the US population (Lellis-Santos et al, 2011). In most cases the symptoms are cold intolerance, fatigue, bradycardia and weight gain. Depending on the cause of hypothyroidism, a goiter, which is the enlargement of the thyroid gland, may develop. In most cases, taking thyroid hormone supplements is an effective treatment. In a healthy individual, the thyroid gland produces the thyroid hormones T3 and T4 through a series of steps. Iodine 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. Iodide in the follicular cell is activated through oxidation by thyroperoxidase (TPO). Active iodide is passively transported into the colloid through an iodide channel on the luminal side of the follicular cell along its concentration gradient. Thyroglobulin (Tg), another precursor of thyroid hormone, enters the colloid through exocytosis from the Golgi complex of the follicular cell. Thyroglobulin (Tg) binds to iodide with the help of thyroperoxidase to form monoiodotyrosine (MIT). If another iodide is added to the resident tyrosine of Tg, di-iodotyrosine (DIT...... middle of paper ......wn is formed. This is how they are stored instead. As a result, people with low thyroid levels hormones undergo weight gain. Furthermore, due to the decrease in fat metabolism, there is a decrease in heat production which also leads to cold intolerance thyroid glands play an important role in regulating cardiac activity. T3 binds to the thyroid hormone receptor alpha in the cell nucleus. T3-TRalpha binds to the thyroid hormone response element, which activates the gene for production of myosin (CITE Hormone ACTION). low levels of thyroid hormones, the beta myosin heavy chain gene is not expressed, resulting in decreased contractility of the heart slow heart rate or bradycardia.