The American Association of Endocrine Surgeons, Patient Education Site

Diseases of thyroid function: Hypothyroidism

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1. Overview

HypothyroidismHypothyroidism - underactive thyroid is a very thyroid problem in which the thyroid is underactive or non-active. It is more often seen in women than men. In general, hypothyroidism can be diagnosed by a thorough history and blood tests.

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2. Symptoms

Symptoms of hypothyroidismHypothyroidism - underactive thyroid may include fatigue, daytime sleepiness, weight gain, water retention, thinning hair, dry skin, constipation, difficulty concentrating, and others.

Hypothyroidism can range from mild forms that are asymptomatic, and found only via blood tests, to severe hypothyroidism, that is associated with significant symptoms along with abnormal lab results.

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3. Diagnosis

The diagnosis of hypothyroidismHypothyroidism - underactive thyroid is achieved through a careful history and physical examination. History is focused on previously noted risk factors such as a personal or family history of autoimmune diseasesAutoimmune disease - when the body's immune system attacks parts of a person's own body., a personal history of head and neck radiation, and a detailed medication history (including non-prescription medications and supplements). Except in cases of longstanding significant hypothyroidism, physical examination will usually not provide any clues to the presence of the disease. The thyroid itself may feel normal.

The TSHTSH - Thyroid stimulating hormone; also known as thyrotropin. The hormone that causes the thyroid to make and release thyroid hormone level is the best test for diagnosing hypothyroidism. A high TSH level confirms the diagnosis of hypothyroidism in most patients. Typically, the free T4 level is usually low.

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4. Common causes

HypothyroidismHypothyroidism - underactive thyroid is most commonly caused by an underlying thyroid disease (i.e. primary hypothyroidism) instead of a problem with TSHTSH - Thyroid stimulating hormone; also known as thyrotropin. The hormone that causes the thyroid to make and release thyroid hormone production (i.e. secondary hypothyroidism). The most common causes of primary hypothyroidism are autoimmune thyroiditis (i.e. Hashimoto's thyroiditisHashimoto's thyroiditis - autoimmune thyroiditis causing hypothyroidism or lymphocytic thyroiditis), surgical removal of the thyroid (i.e. thyroidectomy), radioactive iodine treatment, certain medications, and exposure of the neck to significant radiation.

Autoimmune thyroiditis
Autoimmune thyroiditis is also known as Hashimoto's thyroiditis or chronic lymphocytic thyroiditis, and is the most common cause of hypothyroidism in areas where people get enough iodine in the diet (for example, North America and most of Europe). The majority of patients with Hashimoto's have antibodies to thyroglobulin (Anti-TG AbAnti-TG Ab - Antithyroglobulin antibody; seen in Hashimoto's thyroiditis) and/or thyroid peroxidase (Anti-TPO AbAnti-TPO Ab - Antithyroid peroxidase antibody; seen in Hashimoto's thyroiditis ATC - Anapalstic thyroid cancer ). These antibodies cause destruction of thyroid cells which leads to fewer cells making thyroid hormone. Risk factors for Hashimoto's thyroiditis include female gender, personal history of other autoimmune diseasesAutoimmune disease - when the body's immune system attacks parts of a person's own body., and a family history of autoimmune thyroiditis or other autoimmune diseases.

Removing the whole thyroid will cause hypothyroidism unless the patient is given thyroid hormone replacement postoperatively. In patients who have less than the whole thyroid removed (i.e., subtotal thyroidectomySubtotal thyroidectomy - removal of almost the entire thyroid or lobectomyLobectomy - removal of half the thyroid) the chance of becoming hypothyroid depends on the amount of thyroid removed and any underlying thyroid disease. Even patients who have only had half the thyroid removed are at a higher risk of developing hypothyroidism than the general public.

RAI ablationRAI ablation - the use of radioactive iodine to destroy thyroid cells (either benign or cancer)
Radioactive iodine treatment (RAI treatment) is given to destroy thyroid tissue (especially for Graves' diseaseGraves' disease - autoimmune overproduction of thyroid hormone resulting in hyperthyroidism or thyroid cancer). The higher the dose, the greater the chance of developing hypothyroidism. However, even smaller doses can lead to hypothyroidism, just over a longer time period. Of note, diagnostic radioactive thyroid scans do NOT lead to hypothyroidism.

Abnormal iodine intake
Abnormal iodine intake can lead to hypothyroidism. The major building block of thyroid hormone is iodine, and therefore not enough iodine in the diet can lead to hypothyroidism. Inadequate iodine intake is rare in North America and most of Europe due to the use of iodized salt. The recommended daily iodine intake is approximately 150 mcg per day, and this is the equivalent of approximately 1 tablespoon of iodized table salt. Too much iodine in the diet can overwhelm the thyroid's ability to make thyroid hormone. This is called the Wolff-Chaikoff effect. Most individuals with normally functioning thyroid glands can quickly overcome this problem. However, older patients, those who have undergone a previous thyroidectomy, patients with Hashimoto's thyroiditis, and patients who have had RAIRAI - radioactive iodine treatment may not be able to recover from this problem. In these cases, substances containing a large amount of iodine should be avoided. Specific thing to avoid are dietary supplements, cough medications, the drug amiodarone, and contrast agents used for certain radiology scans (like CAT scans).

Antithyroid medications (Methimazole, Propylthiouracil, etc) are designed to cause hypothyroidism and are typically only used to treat hyperthyroidismHyperthyroidism - overactive thyroid. In these cases, thyroid function needs to be monitored closely. When RAI treatment is used to treat hyperthyroidism, stopping antithyroid medications should be a slow process, and should be overseen by a medical specialist.

Lithium is a medication used in the treatment for certain psychiatric disorders. However, lithium prevents thyroid hormone release and can cause hypothyroidism. The hypothyroidism is usually mild (subclinical). This typically happens within the first two years of starting lithium. Risk factors include female gender, elderly patients, and the presence of underlying autoimmune thyroiditis. Patients on lithium should have their thyroid function closely monitored.

Amiodarone is a potent medication used in the management of certain types of irregular heart rates or arrhythmias. This drug contains iodine. The typical dose of amiodarone contains 40 times the recommended minimum daily intake of iodine. Hypothyroidism can be caused by the Wolff-Chaikoff effect noted above. Patients with autoimmune thyroiditis are at greatest risk for developing amiodarone-induced hypothyroidism.

External radiation
External radiation is most commonly used in the treatment of head and neck cancers (NOT thyroid cancer) and lymphoma. It is an uncommon cause of hypothyroidism. The chance of developing hypothyroidism after external radiation depends on the dose given. External radiation due to nuclear accidents (such as the Chernobyl nuclear reactor incident in 1986) is also a rare cause of hypothyroidism.

Secondary hypothyroidism is a much less common problem. It is caused by diseases that affect the pituitary gland's ability to make and release TSH. Specific problems include pituitary tumors, postpartum pituitary necrosis (Sheehan's syndrome — an uncommon problem where all or part of the pituitary dies after childbirth), trauma, or tumors that grow into the pituitary gland.

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5. Treatment

The treatment of hypothyroidismHypothyroidism - underactive thyroid is replacing the body's natural thyroid hormone with a pill form of thyroid hormone. Thyroid hormone replacement is typically a form of T4T4 - thyroxine thyroid hormone (also known as levothyroxine) because it is longer lasting and only needs to be taken once a day. The shorter lasting T3 may be used in certain cases but needs to be taken 2 to 3 times a day. During treatment, the patient's TSHTSH - Thyroid stimulating hormone; also known as thyrotropin. The hormone that causes the thyroid to make and release thyroid hormone level must be monitored to ensure that the correct dose is given. The goal in most cases is for the TSH to be in the normal range. Typically, the starting dose is based on the patient's weight and is usually 1.6 mcg/kg (2 mcg/kg for patients with thyroid cancer).

There are several situations where the dose may need to be adjusted. For example, in elderly patients, thyroid hormone replacement should be started at a lower dose due to the significant risks of heart problems and increased bone loss. In patients with thyroid cancer, the dose is adjusted to suppress (i.e. lower) TSH below normal levels.

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