Many permanent or temporary conditions can reduce thyroid hormone secretion and cause hypothyroidism. More than 90% of hypothyroidism cases occur from problems that start in the thyroid gland. In such cases, the disorder is called primary hypothyroidism. (Secondary hypothyroidism is caused by disorders of the pituitary gland. Tertiary hypothyroidism is caused by disorders of the hypothalamus.)
The two most common causes of primary hypothyroidism are:
- Hashimoto thyroiditis. This is an autoimmune condition (chronic lymphocytic thyroiditis) in which the body's immune system attacks its own thyroid cells.
- Overtreatment of hyperthyroidism (an overactive thyroid).
Autoimmune Diseases of the Thyroid
Hashimoto thyroiditis, atrophic thyroiditis, and postpartum thyroiditis are all autoimmune diseases of the thyroid. An autoimmune disease occurs when the immune system mistakenly attacks the body's own healthy cells. In the case of autoimmune thyroiditis, a common form of primary hypothyroid disease, the cells under attack are in the thyroid gland and include, in particular, a thyroid protein called thyroid peroxidase. The autoimmune disease process results in the destruction of thyroid cells.
The most common form of hypothyroidism is Hashimoto thyroiditis, a genetic disease named after the Japanese doctor who first described thyroid inflammation (swelling of the thyroid gland). Women are more likely than men to develop this disease.
An enlargement of the thyroid gland, called a goiter, is almost always present and may appear as a growth in the neck. Hashimoto thyroiditis is permanent and requires lifelong treatment. Both genetic and environmental factors appear to play a role in its development.
The other main type of autoimmune thyroid disease is Graves disease, which causes hyperthyroidism (overactive thyroid).
Atrophic thyroiditis is similar to Hashimoto thyroiditis, except a goiter is not present.
Riedel thyroiditis is a rare autoimmune disorder, in which scar tissue progresses in the thyroid until it produces a hard, stony mass that suggests cancer. Hypothyroidism develops as the scar tissue replaces healthy tissue. Surgery is usually required, although early stages may be treated with corticosteroids or other immunosuppressive drugs.
Autoimmune Thyroiditis Due to Pregnancy
Hypothyroidism may also occur in women who develop antibodies to their own thyroid during pregnancy, causing inflammation of the thyroid after delivery.
Subacute thyroiditis is a temporary condition that passes through 3 phases: hyperthyroidism, hypothyroidism, and a return to normal thyroid levels.
People may have symptoms of both hyperthyroidism and hypothyroidism (such as rapid heartbeat, nervousness, and weight loss, along with or followed by depression and fatigue), and they can feel extremely sick. Symptoms last about 6 to 8 weeks and then usually resolve, although each form carries some risk for becoming chronic.
The 3 forms of subacute thyroiditis follow a similar course.
Painless Postpartum Subacute Thyroiditis
Postpartum thyroiditis is an autoimmune condition that occurs in up to 10% of pregnant women and tends to develop between 4 to 12 months after delivery. In most cases, the woman develops a small, painless goiter. This condition is generally self-limiting and requires no therapy unless the hypothyroid phase is prolonged. If so, therapy may be thyroxine replacement for a few months. A beta-blocker drug may also be recommended if the hyperthyroid phase needs treatment. About 20% of women with this condition go on to develop permanent hypothyroidism.
Painless Sporadic, or Silent Thyroiditis
This painless condition is very similar to postpartum thyroiditis, except it can occur in both men and women and at any age. About 20% of people with silent thyroiditis may develop chronic hypothyroidism. Treatment considerations are the same as for postpartum subacute thyroiditis. Some medications may cause this type of thyroiditis.
Painful, or Granulomatous Thyroiditis
Subacute granulomatous thyroiditis, also called de Quervain thyroiditis, comes on suddenly with mild to severe neck pain and swelling, and often occurs several weeks after flu-like symptoms. It is thought to be caused by a viral infection and generally occurs in the summer. It is much more common in women than men and is usually a temporary condition. Treatments typically include pain relievers and, in severe cases, corticosteroids or beta-blockers.
After Treatment of Hyperthyroidism
Many people who receive radioactive iodine treatments for an overactive thyroid develop permanent hypothyroidism within a year of therapy. Radioactive iodine is a common treatment for Graves disease, which is the most common form of hyperthyroidism, an autoimmune condition resulting in excessive secretion of thyroid hormones.
After treatment for Graves disease, many people gradually develop hypothyroidism and need to take thyroid hormones for the rest of their lives. Other types of treatment for overactive thyroid glands (such as anti-thyroid drugs or surgery) may also result in hypothyroidism.
Too much or too little iodine can cause hypothyroidism. If there is a deficiency of iodine, the body cannot manufacture thyroxine (T4). Millions of people around the world have hypothyroidism because of insufficient iodine in their diets. This global public health issue used to be even more widespread, but has now been almost completely resolved due to salt iodization programs. Too much iodine is a signal to inhibit the conversion process of T4 to T3. The end result in both cases is inadequate production of thyroid hormones. Some evidence suggests that excess iodine may trigger the process leading to Hashimoto thyroiditis.
People who have complete removal (total thyroidectomy) of the thyroid gland to treat thyroid cancer need lifetime treatment with thyroid hormone. Removing one of the two lobes of the thyroid gland (hemithyroidectomy), usually because of benign growths on the thyroid gland, rarely produces hypothyroidism. The remaining thyroid lobe will generally grow so that it can produce sufficient amounts of thyroid hormone for normal function. However, to prevent the formation of additional nodules, many doctors recommend thyroid hormone treatment.
People with Graves disease who have surgery to remove most of both thyroid lobes (subtotal thyroidectomy) may develop hypothyroidism.
Drugs and Medical Treatments that Reduce Thyroid Levels
A drug used to treat bipolar disorder, has multiple effects on thyroid hormone synthesis and secretion. Many people treated with lithium go on to develop hypothyroidism and some develop a goiter. Most people develop subclinical hypothyroidism, but a small percentage experience overt hypothyroidism.
Is used to treat abnormal heart rhythms, contains high levels of iodine and can induce hyper- or hypothyroidism, particularly in people with existing thyroid problems.
Drugs used for treating epilepsy, such as phenytoin and carbamazepine, can reduce thyroid hormone levels. Interferons and interleukins, which are used to treat hepatitis, multiple sclerosis, and other conditions, can induce either hypothyroidism or hyperthyroidism. Some drugs used in cancer chemotherapy, such as sunitinib (Sutent) or imatinib (Gleevec), can also cause or worsen hypothyroidism.
High-dose radiation for cancers of the head or neck and for Hodgkin disease can cause hypothyroidism up to 10 years after treatment.
Other Medical Conditions
Several medical conditions involve the thyroid and can change the normal gland tissue so that it no longer produces enough thyroid hormone. Examples include hemochromatosis, scleroderma, and amyloidosis.
Causes of Secondary and Tertiary Hypothyroidism
In rare instances, usually due to a tumor, the pituitary gland will fail to produce TSH, the hormone that stimulates the thyroid to produce its hormones. In such cases, the thyroid gland shrinks and secondary hypothyroidism occurs.
Causes of Hypothyroidism in Infants
Hypothyroidism in newborns (known as congenital hypothyroidism) occurs in one in every 3,000 to 4,000 births. It usually persists throughout life.
Permanent Congenital Hypothyroidism
In most cases of permanent congenital hypothyroidism, the thyroid gland is missing, underdeveloped, or not properly located. In other cases, hormone production is impaired or the pituitary or hypothalamus glands function abnormally. Genetic abnormalities may be a factor in congenital hypothyroidism, but in many cases the cause is unknown.
Temporary Hypothyroidism in Infants
Temporary hypothyroidism can also occur in infants. Possible causes include various immunologic, environmental, and genetic factors, including the following:
- Women who have an underactive (low) thyroid, including those who develop the problem during pregnancy, are at increased risk for delivering babies with congenital (newborn) hypothyroidism. Maternal hypothyroidism can also cause premature delivery and low birth weight.
- Some of the drugs used to treat hyperthyroidism (overactive thyroid) block the production of thyroid hormone. These same drugs can also cross the placenta and cause hypothyroidism in the infant.
- If a pregnant woman has untreated hyperthyroidism, her newborn infant may have hypothyroidism for a short period of time. This is because the excess thyroid hormone in the woman's blood crosses the placenta and signals the fetus not to produce as much of its own thyroid hormone.
- Iodine deficiency may cause temporary hypothyroidism. (Exposure to too much iodine immediately after birth, for example from iodine-containing disinfectants or medicines, can also cause thyroid dysfunction.)
- Premature birth increases the risk of temporary hypothyroidism in the infant.
Children with temporary congenital hypothyroidism should be followed up regularly during adolescence and adulthood for possible thyroid problems. The risk for future thyroid problems is highest when girls born with this condition reach adulthood and become pregnant.