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A thyroid disease is a medical condition that affects the function of the thyroid gland. The thyroid gland's primary role is to produce thyroid hormones -- triiodothyronine (T3) and thyroxine (T4). These hormones regulate the body's metabolic rate, and increase protein synthesis. The thyroid gland thus plays an important role in growth and maturation in children and helps multiple different organs function in adults.

The symptoms and signs of thyroid disease can vary depending on the type of disease. There are five general types of thyroid dysfunction: 1) hypothyroidism (low function), caused by a deficiency of thyroid hormones, 2) hyperthyroidism or thyrotoxicosis (high function), caused by an excess of thyroid hormones, 3) structural abnormalities, most commonly goiter which is an enlargement of the thyroid gland, 4) tumor, which can be benign or malignant and manifest as a thyroid nodule, and 5) abnormal thyroid function tests without any clinical symptoms.

These diseases have a large range of symptoms and can affect many different ages. Common hypothyroid symptoms may include fatigue, low energy, weight gain, inability to tolerate the cold, slow heart rate, dizziness, depression, dry skin and constipation, while hyperthyroid symptoms may include irritability, weight loss, a fast heartbeat, heat intolerance, diarrhea, and enlargement of the thyroid. In both hypothyroidism and hyperthyroidism, there may be swelling of a part of the neck, which is also known as goiter.

Hypothyroidism affects 3-10% percent of adults, with a higher incidence in women and the elderly. An estimated one-third of the world's population currently lives in areas of low dietary iodine levels, making iodine-deficiency the most common cause of hypothyroidism and endemic goiter. In regions of severe iodine deficiency, the prevalence of goiter is as high as 80%. In areas where iodine-deficiency is not found, the most common thyroid disease is autoimmune thyroid disease, namely Hashimoto's thyroiditis (a type of hypothyroidism) with a prevalence of 1-2%. As for hyperthyroidism, Grave's disease, an autoimmune condition, is the most common type with a prevalence of 0.5% in males and 3% in females.

Although thyroid nodules are common, thyroid cancer itself is actually rare. Thyroid cancer accounts for <1% of all registered malignancies in the UK, though it is the most malignant endocrine tumor by making up >90% of all cancers of the endocrine glands.

Diagnosis of thyroid disease can often be made through several laboratory tests. The first is thyroid-stimulating hormone (TSH), which is generally below normal in hyperthyroidism and above normal in hypothyroidism. The other useful laboratory test is non-protein-bound thyroxine or free T4. Total and free triiodothyroxine (T3) levels are less used. Anti-thyroid antibodies can also be used, where elevated anti-thyroglobulin and anti-thyroid peroxidase antibodies are commonly found in hypothyroidism from Hashimoto thyroiditis and TSH-receptor antibodies are found in hyperthyroidism caused by Grave's disease. Procedures such as

Imbalances in the production of thyroid hormones (T4 and T3) arise from dysfunction of either 1) the thyroid gland itself, 2) the pituitary gland, which produces, or 3) the hypothalamus, which regulates the pituitary gland via thyrotropin-releasing hormone (TRH).

Thyroid Disease Page Edits - 11/30/2016:

BLOOD TESTS

Thyroid Function Tests (TFTs)

Further information: Thyroid function tests

There are several hormones that can be measured in the blood to determine how the thyroid gland is functioning. These include the thyroid hormones triiodothyronine (T3) and its precursor thyroxine (T4), which are produced by the thyroid gland. Thyroid-secreting hormone (TSH) is another important hormone that is secreted by the anterior pituitary cells in the brain. Its primary function is to increase the production of T3 and T4 by the thyroid gland.

The most useful marker of thyroid gland function is serum thyroid-stimulating hormone (TSH) levels. TSH levels are determined by a classic negative feedback system in which high levels of T3 and T4 suppress the production of TSH, and low levels of T3 and T4 increase the production of TSH. TSH levels are thus often used by doctors as a screening test, where the first approach is to determine whether TSH is elevated, suppressed, or normal. Because a single abnormal TSH level can be misleading, T4 and T3 levels must be measured in the blood to further confirm the diagnosis. When circulating in the body, T4 and T3 are bound to transport proteins, namely thyroxine-binding globulin, transthyretin and albumin. Only a small fraction of the circulating thryoid hormones are unbound or free, and thus biologically active. T3 and T4 levels can thus be measured as free T3/T4 and total T3/T4, which takes into consideration the free T3/T4 in addition to the protein-bound T3/T4. Free T3 and T4 measurements are important because certain drugs and illnesses can affect the concentrations of transport proteins, resulting in differing total and free thyroid hormone levels. There are differing guidelines for free T4 measurement. Antithyroid Antibodies
 * Elevated TSH levels can signify inadequate thyroid hormone production (hypothyroidism)
 * Suppressed TSH levels can point at excessive thyroid hormone production (hyperthryoidism)
 * Free T4 levels are often added to determine the degree of hypothyroidism when disease is suspected in the hypothalamus or pituitary gland
 * Free T4 levels may also be tested in patients who have convincing symptoms of hyper- and hypothyroidism, despite a normal TSH
 * Autoantibodies may be detected in various disease states (anti-TG, anti-TPO, TSH receptor stimulating antibodies).
 * There are two cancer markers for thyroid derived cancers. Thyroglobulin (TG) for [null well differentiated] papillary or follicular adenocarcinoma, and the rare medullary thyroid cancer has calcitonin as the marker.
 * [null Very    infrequently], TBG and transthyretin levels may be abnormal; [null these] are     not routinely tested.
 * To differentiate between different types of    hypothyroidism, a specific test may be used. Thyrotropin-releasing hormone (TRH) is injected into the body through a vein. This hormone [null is naturally secreted     by] the hypothalamus and stimulates the pituitary gland. The     pituitary responds by releasing thyroid -stimulating hormone (TSH). Large     amounts of externally administered TRH can suppress the subsequent release     of TSH. This amount of release-suppression is exaggerated in primary     hypothyroidism, major depression, cocaine dependence, [null amphetamine dependence and] chronic     phencyclidine abuse. There is a failure to suppress in the manic phase of     bipolar disorder.