What are thyroid disorders?
The thyroid gland normally weighs about twenty to thirty-five grams and is located in the neck just below the larynx, or voice box. The gland is named for the shield-shaped “thyroid” cartilage that forms the front of the larynx. The thyroid has two lateral lobes that are connected by an isthmus that crosses in front of the trachea. By placing a finger on the trachea below the larynx it is possible to feel the ridge-like isthmus pass under the finger after swallowing. The bilobed (two-lobed) shape of the rest of the gland can be felt just under the skin of the neck on either side of the midline, although its boundaries are normally indistinct except to a trained examiner.
The thyroid produces two major hormones. Thyroxine, a product of the follicular cells, is the major hormone produced by the thyroid that helps regulate metabolism. Within the thyroid are also parafollicular cells that produce calcitonin, an essential hormone involved in calcium metabolism. In the tissue of the thyroid are also embedded two pairs of parathyroid glands. The parathyroid glands produce parathyroid hormone, which is required to maintain normal levels of blood calcium. In the case of thyroid surgery, it is important that the parathyroid glands are not damaged or removed; otherwise, there may be life-threatening tetanus—the sustained contraction of muscles, including those needed for breathing.
The normal functioning of the thyroid results from an elaborate physiological control system involving the hypothalamus of the brain, the anterior lobe of the pituitary gland, and the thyroid gland. The hypothalamus produces thyrotropic-releasing hormone (TRH), which is passed by special blood vessels to the anterior lobe of the pituitary, the adenohypophysis. The TRH-stimulated cells in the adenohypophysis produce thyroid-stimulating hormone (TSH), which is released into the general circulation. When it reaches the thyroid gland, it stimulates the gland to produce thyroxine. Normally, thyroxine has a negative feedback effect on its own production; that is, thyroxine can inhibit the activity of the hypothalamus and the pituitary to maintain its concentration in the blood. Various thyroid disorders, which are more common in women than in men, can develop from tumors that either increase or decrease the hormones produced in these three interdependent structures.
The normal thyroid (or euthyroid state) produces mainly thyroxine, which is converted into triiodothyronine in the tissues of the body before it has its effects, which are generally to increase the metabolic rate of the body. Some triiodothyronine is directly produced by the thyroid. The thyroxine molecule contains iodide, the negative ion of iodine; iodine is therefore an essential component of one’s diet. If iodine is not available in the diet—as in the case of vegetables grown in geographical areas glaciated in the past, such as mountainous terrain and the American Midwest—then the body cannot produce thyroxine. Industrialized countries have iodine added to table salt to ensure an adequate supply of this element in the diet. A lack of iodine, and therefore a lack of thyroxine, prevents the functioning of the negative feedback effect of thyroxine on the hypothalamus and pituitary, resulting in very low thyroxine levels and high TSH levels in the blood. High levels of TSH cause substantial growth of the thyroid, which will bulge from the neck as a goiter. A person with such a condition would be hypothyroid (that is, have lower-than-normal thyroxine levels in the blood) and may be affected by cretinism (mental impairment and stunted physical growth) if this condition occurs early in childhood.
Hypothyroidism can arise in other ways as well. Hashimoto’s thyroiditis is a common type of hypothyroidism that is caused by an autoimmune reaction whereby white blood cells known as lymphocytes infiltrate the thyroid and gradually destroy its tissue. The presence of antibodies against normal thyroid proteins can be detected with this condition. The usual signs of hypothyroidism are an intolerance of cold, a low body temperature, a lower rate of metabolism, a tendency to sleep longer, a general lack of energy, infrequent bowel movements, constipation, possible weight gain, a puffy face and hands, a slow heart rate, cold and scaly skin, a lack of perspiration, and possible emotional withdrawal and depression.
Graves’ disease , the most common type of hyperthyroidism, is an autoimmune disorder in which antibodies mimic the action of TSH and therefore stimulate the thyroid to produce excessive thyroxine. Sometimes, nodules develop in the thyroid that may produce the excessive thyroxine. Although the presence of a nodule in the thyroid may cause a person to suspect cancer, the nodules are usually benign. Hyperthyroidism may be associated with bulging eyes, but this orbitopathy does not always occur. Generally, there is an intolerance of heat, a loss of body weight, a high degree of nervousness, increased or decreased skin pigmentation, more frequent bowel movements, loss of hair, and a very rapid heart rate.
Patients suspected of having hypothyroidism or hyperthyroidism will have their blood tested for levels of TSH and thyroxine. Ultrasonography can be used to detect tumors and serve as an anatomical guide for potential surgery. Hypothyroidism patients are prescribed a small oral dose (less than 1 milligram per day) of thyroxine, which is adjusted until a euthyroid state is obtained within a few months. Then the patient is maintained on thyroxine, with perhaps yearly checkups by a physician. For hyperthyroidism patients, several modes of treatment are possible. Antithyroid drugs, such as propylthiouracil (PTU) or methimazole, can be given to inhibit thyroxine synthesis. Radioactive iodine is commonly given to destroy part of the thyroid gland and thus reduce its thyroxine output. Second or even third doses of radioactive iodine may be given if the blood thyroxine levels remain high. Radioactive iodine is not used during pregnancy because damage to the fetal thyroid is likely. Additionally, surgery can be performed to remove enough thyroid tissue to restore normal thyroxine levels. Following any of the treatments, a hypothyroidism may be induced that will require that the patient receive thyroxine supplements. Finally, surgery can be used to reduce the bulging of the eyes caused by hyperthyroidism.
Bar, Robert S. Early Diagnosis and Treatment of Endocrine Disorders. Totowa, N.J.: Humana Press, 2003.
Braverman, Lewis E., ed. Diseases of the Thyroid. 2d ed. Totowa, N.J.: Humana Press, 2003.
Health Library. "Hyperthyroidism." Health Library, November 26, 2012.
Health Library. "Hypothyroidism." Health Library, March 15, 2013.
Hershman, Jerome M., ed. Endocrine Pathophysiology: A Patient-Oriented Approach. 3d ed. Philadelphia: Lea & Febiger, 1988.
Kovacs, William J.., and Sergio R. Ojeda, eds. Textbook of Endocrine Physiology. 6th ed. New York: Oxford University Press, 2012.
MedlinePlus. "Thyroid Diseases." MedlinePlus, May 30, 2013.
Melmed, Shlomo, and Robert Hardin Williams, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia: Elsevier/Saunders, 2011.
Ruggieri, Paul, and Scott Isaacs. A Simple Guide to Thyroid Disorders: From Diagnosis to Treatment. Omaha, Nebr.: Addicus Books, 2010.
Surks, Martin I. The Thyroid Book. Rev. ed. Yonkers, N.Y.: Consumer Reports Books, 1999.