Causes of Underactive Thyroid

Surgical Removal of the Thyroid
Part or all of the thyroid gland may be surgically removed as a treatment for:
  • Hyperthyroidism (when the thyroid makes too much thyroid hormone)
  • A large goiter, which is an enlarged thyroid gland that may cause the neck to appear swollen and can interfere with normal breathing and swallowing
  • Thyroid nodules, which are lumps in the thyroid that can produce excess thyroid hormone
  • Thyroid cancer.
When part of the thyroid is removed, the remaining part may produce normal amounts of thyroid hormone, but some people who have this surgery develop hypothyroidism. Removal of the entire thyroid always results in hypothyroidism within two to four weeks.
Radiation Treatment of the Thyroid
Radioactive iodine, a common treatment for an overactive thyroid, gradually destroys the cells of the thyroid. Almost everyone who receives this treatment eventually develops hypothyroidism. People with Hodgkin's disease, other lymphomas, and head or neck cancers often are treated with radiation, which can also damage the thyroid.
Thyroiditis causes stored thyroid hormone to leak out of the inflamed thyroid gland. At first, the leakage raises hormone levels in the blood, leading to hyperthyroidism that lasts for a month or two. Most people then develop hypothyroidism before the thyroid is completely healed.
Several types of thyroiditis can cause an underactive thyroid:
  • Subacute thyroiditis. This condition involves painful inflammation and enlargement of the thyroid. Healthcare providers aren't sure what causes subacute thyroiditis, but it may be related to a viral or bacterial infection. The condition usually goes away on its own in a few months.
  • Postpartum thyroiditis. Up to 10 percent of women who have been pregnant develop postpartum thyroiditis within a few months of giving birth. Most of the time, it is a temporary condition. In some women, however, the thyroid does not heal and their hypothyroidism is permanent. Postpartum thyroiditis is believed to be an autoimmune condition.
  • Silent thyroiditis. This type of thyroiditis is called "silent" because it is painless, as is postpartum thyroiditis, even though the thyroid may be enlarged. Silent thyroiditis is probably an autoimmune condition and sometimes develops into permanent hypothyroidism.
Congenital Hypothyroidism
Some babies are born with a thyroid that is not fully developed or does not function properly. If left untreated, congenital hypothyroidism can lead to mental retardation and growth failure. Most newborns in the United States are screened for an underactive thyroid, and early treatment can prevent these complications.
Last reviewed by: Arthur Schoenstadt, MD
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