A thyroid disorder is any condition affecting the thyroid gland. The thyroid is a small gland shaped like a butterfly and located in the front part of the neck beneath the voice box (larynx). A healthy thyroid cannot usually be felt through the skin.

The thyroid gland influences many functions of the body, such as metabolism, blood pressure, heart rate, digestion, body temperature, growth, reproductive functions and development of the central nervous system. These body functions depend on two hormones released from the thyroid gland: thyroxine (known as T4) and triiodothyronine (T3). These thyroid hormones, in turn, depend on small amounts of iodine, a mineral present in shellfish and in agricultural products grown in iodine-rich soil. Because iodine is important to body functioning, iodized salt was introduced in the United States in the 1920s to ensure adequate dietary intake of iodine.

Damage to the thyroid gland, or an imbalance or disruption of the chemical processes that involve the thyroid can result in a thyroid disorder.

Thyroid disorders are common. Although estimates vary, experts believe about 20 million Americans are affected by some type of thyroid disorder. In addition, many people are believed to have undiagnosed thyroid disorders. The two most common thyroid disorders are hypothyroidism (too little thyroid hormone) and hyperthyroidism (too much thyroid hormone). The United States National Health and Nutrition Examination Survey (NHANES III) found that about 4.6 percent of adults had hypothyroidism and 1.3 percent had hyperthyroidism.

Left untreated, thyroid disorders can have serious consequences for the heart. Certain thyroid disorders, such as Hashimoto's thyroiditis (an autoimmune disease that inflames the thyroid and causes hypothyroidism), have been associated with an increased risk of high cholesterol levels (hypercholesterolemia), high blood pressure, hardening of the arteries (atherosclerosis), heart disease, heart attack or stroke. Atrial fibrillation has also been closely linked to thyroid disease. Even subtle deviations in thyroid hormone levels can affect the heart.

Hypothyroidism can cause numerous body processes, including heart function, to slow. This can cause the heart to become weak in the contraction (systolic) and relaxation (diastolic) phase. It can also cause the linings of the blood vessels to become stiff. The result is that the heart cannot pump as strongly as it should. When the heart does not relax between beats, diastolic dysfunction may occur. This is a serious condition that causes blood to build up in the body's organs primarily the lungs, where it is also known as pulmonary congestion.

Other cardiac complications of hypothyroidism include an enlarged heart, in which the heart muscle expands in response to the added stress placed upon it. This can, in turn, can lead to heart failure. Complications may also include pericardial effusion and elevated homocysteine, which is associated with an increased risk of heart attack.

Hyperthyroidism can cause stronger-than-normal heart contractions and an increased heart rate. The heart must work harder, which can create greater problems for people with heart disease. The output of blood by the heart increases and blood pressure within the ventricles becomes elevated.

Other cardiac complications of hyperthyroidism include atrial fibrillation. This may increase the risk of blood clots that may lead to stroke or pulmonary embolism. Cardiac arrest and sudden cardiac death may also result from the increased heart rate that occurs with hyperthyroidism. Additional complications may include mitral valve prolapse and pulmonary hypertension.

Additional complications of thyroid disorders include thyrotoxicosis (also called thyroid crisis or thyroid storm), where patients with hyperthyroidism experience a sudden intensification of their symptoms. This can cause abdominal pain, sudden confusion and severe agitation, high fever and coma. Thyroid disorders may also lead to infertility, harm a developing fetus and increase the risk of miscarriage during pregnancy.

In some cases, thyroid disorders require lifelong treatment.