What Causes an Underactive Thyroid?

The thyroid is a gland in the endocrine system of the body. Endocrine glands affect every organ, cell and function of the human body. These glands regulate growth and development, tissue function, sexual function, metabolism, mood and the reproductive processes.
The thyroid plays an essential role in the endocrine system by producing the thyroid hormones, thyroxine (T4) and triiodothyronine (T3), which govern the rate that cells produce energy by the burning of fuels from food. Thyroxine is active in the development of the brain and the nervous systems and in bone growth. Triiodothyronine affects almost every physiological process including growth and development, heart rate, metabolism and temperature of the body.

The pituitary gland acts as a thermostat that controls the secretion of hormones from the other glands. The pituitary secretes thyrotropin which regulates the release of thyroid hormones. The thyroid in turn releases the proper amounts of T4 and T3.

If interaction between the pituitary and thyroid glands is interrupted, the thyroid produces less or more thyroid hormones than the body needs to remain healthy. Symptoms may appear slowly, over a number of months and years, as thyroid function is gradually impaired.
Symptoms of low thyroid hormones, or hypothyroidism, include lack of energy, hair loss, low metabolism (weight gain), insomnia, constipation, low blood pressure, dry skin and sensitivity to cold. Any one of these symptoms alone does not indicate hypothyroidism, but several symptoms at the same time may need to be checked out by a physician.
A moderate lack of thyroid hormones is not in itself dangerous, but as the disease progresses, treatment is necessary to prevent a serious health problem and even death.

There are several causes of an underactive thyroid.
The most common cause is Hashimoto’s thyroiditis, an autoimmune disease in which the body’s own immune system attacks thyroid tissue resulting in inflammation of the thyroid.

A thyroidectomy, or removal of all or a part of the thyroid, can result in low thyroid hormones. If a portion of the thyroid remains, it is possible the intact portion will take over and produce enough thyroid hormones. But it is likely that, over time, replacement thyroid hormones will be needed.
Therapy for cancer, in addition to thyroid cancer, can kill the thyroid with resultant hypothyroidism.
Malfunction of the pituitary gland adversely affects production of thyroid hormones.
Treatment to stop a goiter from growing can kill all or part of the thyroid. 
Severe iodine deficiency decreases the thyroid’s ability to function properly.
Congenital hypothyroidsm affects infants at birth.

Treatments to reduce the production of thyroid hormones in the case of hyperthyroidism can initiate hypothyroidism.
Thyroid disorders are more common in women than in men. If left untreated hypothyroidism can result in heart disease. Severe hypothyroidism can lead to myxedema coma, a rare but dangerous disease.
Blood tests are required to determine if a patient is low on thyroid hormones and if so, the physician will prescribe and oversee the administration of replacement thyroid hormones such as levothyroxine, until the correct level of dosage has been reached.