What are thyroid diseases?
Thyroid diseases are primarily conditions that affect the amount of thyroid hormones being produced. The American Thyroid Association estimates that 20 million Americans have some form of thyroid disease, and approximately 60% of those with thyroid disease do not know it. Women are more likely than men to have thyroid problems, with 1 in 8 developing a thyroid disorder during her life. The following is a list of the more common thyroid disorders.
Diseases that present as abnormal thyroid function:
- Hypothyroidism: too little thyroid hormone; slowing of body functions; symptoms include weight gain, dry skin, constipation, cold intolerance, puffy skin, hair loss, fatigue, and menstrual irregularity in women. Severe untreated hypothyroidism, called myxedema, can lead to heart failure, seizures, and coma. In children, hypothyroidism can stunt growth and delay sexual development. Specific types of hypothyroidism include:
- Congenital hypothyroidism: this condition affects infants from birth; it is caused by inadequate thyroid hormone and is most commonly due to a thyroid gland that is missing, only partially developed, or located in an abnormal part of the body. The rest of the cases are due to a normal-sized or enlarged thyroid that does not function properly or produce sufficient thyroid hormone. Left untreated, this condition can cause delays in physical and intellectual development. Hypothyroidism testing is performed in the United States as part of newborn blood screening programs since early detection and treatment can minimize long-term damage.
- Hashimoto thyroiditis: the most common cause of hypothyroidism in the United States; it is a chronic autoimmune condition in which the immune response targets the thyroid, causing inflammation and damage and the production of autoantibodies. With Hashimoto thyroiditis, however, the thyroid produces low amounts of thyroid hormone.
- Iodine deficiency: lack of this element (as iodide) diminishes the ability of the thyroid gland to make enough thyroid hormone. T4 has four iodides and T3 has three. Iodide is present throughout the environment, but most iodide is in sea water and seaweed. People who live in countries with little access to the sea often have iodide deficiency unless their sources of food are supplemented with iodide. Fortunately, iodide is used to fight bacterial growth in many foods (such as iodized salt) and is also in many dietary supplements.
- Hyperthyroidism: too much thyroid hormone; sometimes called "overactive thyroid;" acceleration of body functions; symptoms include increased heart rate, anxiety, weight loss, difficulty sleeping, tremors in the hands, weakness, and sometimes diarrhea. There may be puffiness around the eyes, dryness, irritation, and, in some cases, bulging of the eyes. The affected person may experience light sensitivity and visual disturbances. Because the eyes may not move normally, the person may appear to be staring. Specific types of hyperthyroidism include:
- Graves disease: the most common cause of hyperthyroidism; it is a chronic autoimmune disorder in which the affected person's immune system produces antibodies that act like TSH, stimulating the thyroid to produce excessive amounts of thyroid hormone.
- Thyroid tumor: a small benign tumor may become insensitive to the negative feedback of low TSH and continue to produce excess thyroid hormone.
- Abnormal thyroid stimulation: a tumor of the cells that produce TSH can result in excess thyroid hormone production. Also, human chorionic gonadotropin (hCG), the hormone that supports the growth of the fetus in pregnancy, can act like TSH and sometimes produce hyperthyroidism in pregnant women, especially if their hCG levels are very high.
- Goiter: a visible enlargement of the thyroid gland; in the past, this condition was relatively common and was due to iodine deficiency but, with iodine supplementation of food, the incidence of dietary-related goiters has declined significantly in the U.S. In other parts of the world, however, iodine-related goiters are still common and represent the most common cause of hypothyroidism in some countries. Goiters may compress vital structures of the neck, including the trachea and esophagus. This compression can make it difficult to breathe and swallow. Any of the diseases listed above can cause goiters. A rare cause is thyroid hormone resistance syndrome, in which a mutation in the thyroid hormone receptor decreases thyroid hormone function.
- Thyroiditis: an inflammation of the thyroid gland; it may be associated with either hypo- or hyperthyroidism. It may be painful, feeling like a sore throat, or painless. Thyroiditis may be due to an autoimmune disorder (especially Hashimoto thyroiditis), an infection, exposure to a chemical that is toxic to the thyroid, or due to an unknown cause (idiopathic). Depending on the cause, it can be acute but transient or chronic.
Diseases that present as thyroid tumors:
- Thyroid nodule: a small lump on the thyroid gland that may be solid or a fluid-filled cyst; these nodules are common and the overwhelming majority of them are harmless. Occasionally, however, thyroid nodules can be cancerous and need to be treated.
- Thyroid cancer: thyroid cancer is fairly common and its incidence has been increasing over the past few decades. The American Cancer Society estimates that about 63,000 new cases will be diagnosed in the U.S. in 2014, making it the 8th most common type of cancer and the fastest growing type. There are four main types of thyroid cancers:
- Papillary thyroid cancer—about 80% of thyroid cancer cases are papillary. This type affects more women than men and is more common in younger people.
- Follicular thyroid cancer—about 15% of thyroid cancers are follicular, a more aggressive type of cancer that tends to occur in older women.
- Anaplastic thyroid cancer, also found in older women, accounts for about 2% of thyroid cancers and tends to be both aggressive and difficult to treat.
- Medullary thyroid cancer (MTC)—accounts for 3% of thyroid cancers and is malignant; it can spread beyond the thyroid and be difficult to treat if it is not discovered early. MTC produces excess calcitonin and may be found alone or linked with other endocrine cancers in a syndrome called multiple endocrine neoplasia syndrome. The cells that make calcitonin are different from the cells that make thyroid hormone. These may grow in number, resulting in a disorder called C-cell hyperplasia. This is a benign condition that also produces excess calcitonin; it may or may not progress to become medullary thyroid cancer.
- Lymphoma, a tumor composed of lymphocytes (the cells that produce immunity from bacteria and viruses), can also occur in the thyroid.