What causes an underactive thyroid

An underactive thyroid, or hypothyroidism, happens when your thyroid gland does not make enough thyroid hormone. The most common cause worldwide is iodine deficiency—your thyroid needs iodine to produce the hormones T3 and T4. In countries with iodized salt, however, the leading cause is Hashimoto's thyroiditis, an autoimmune disease in which your immune system attacks thyroid tissue and gradually destroys it. Other causes include surgical removal of the thyroid, radiation to the neck or chest, certain medications, and less commonly, problems with the pituitary gland that controls thyroid function.

The reason matters because it shapes what happens next. If iodine deficiency is the cause, adding iodine can restore function. If Hashimoto's is the cause, you will need lifelong hormone replacement because the immune damage is permanent. Understanding which cause applies to you helps your doctor predict how your condition will progress and what monitoring you will need.

Key Takeaways

  • Hashimoto's thyroiditis, an autoimmune attack on the thyroid, is the most common cause of hypothyroidism in iodine-sufficient countries.
  • Iodine deficiency remains the leading preventable cause of hypothyroidism worldwide, though it is rare in countries that use iodized salt.
  • Surgical removal of the thyroid and radiation to the neck or chest are common medical causes that develop after treatment for other conditions.
  • Some medications, including lithium and certain heart and cancer drugs, can trigger hypothyroidism as a side effect.
  • Problems with the pituitary gland can prevent it from signaling the thyroid to work, causing secondary hypothyroidism.

Hashimoto's thyroiditis: autoimmune attack on the thyroid

In Hashimoto's thyroiditis, your immune system produces antibodies that target thyroid peroxidase (TPO) and thyroglobulin, proteins the thyroid needs to make hormones. Over months or years, these antibodies destroy thyroid tissue. The damage is usually permanent, though the rate of decline varies—some people's thyroid function drops quickly, while others decline slowly over a decade.

Hashimoto's is more common in women than men and often runs in families, suggesting both genetic and environmental factors play a role. It can develop at any age but is most common in middle age. Blood tests can detect the antibodies (anti-TPO and anti-thyroglobulin) that signal Hashimoto's, which helps distinguish it from other causes. Once diagnosed, the condition is managed with thyroid hormone replacement, not by treating the immune system itself.

Iodine deficiency and thyroid function

Your thyroid uses iodine as a raw material to build T3 and T4 hormones. Without enough iodine, the thyroid cannot produce adequate hormone no matter how hard it tries. In regions where iodine is scarce in soil and food—parts of Asia, Africa, and Eastern Europe—iodine deficiency remains the leading cause of hypothyroidism and the most common preventable cause of intellectual disability in children.

In North America, Europe, and Australia, iodine deficiency is now rare because salt is routinely iodized and seafood and dairy products contain iodine. When hypothyroidism does develop in these regions, iodine deficiency is an uncommon cause. However, people who avoid salt entirely, follow strict vegan diets without iodine-rich foods, or live in areas with very low soil iodine can still develop deficiency. Adding iodine through diet or supplementation can restore thyroid function if caught before permanent damage occurs.

Thyroid surgery and radiation as causes

Surgical removal of part or all of the thyroid—performed to treat thyroid cancer, nodules, or Graves' disease—directly reduces the amount of tissue available to produce hormone. The extent of hypothyroidism depends on how much thyroid was removed. Partial removal may cause mild hormone deficiency, while total removal guarantees hypothyroidism requiring lifelong replacement therapy.

Radiation to the neck or chest, whether from cancer treatment, head and neck radiation, or (rarely) nuclear accidents, damages thyroid cells and reduces their ability to produce hormone. The effect may appear months or years after radiation. Radiation can also trigger Hashimoto's thyroiditis as a secondary effect. People who received radiation in childhood face higher risk because the thyroid has decades ahead to develop problems.

Medications that can cause hypothyroidism

Several medications interfere with thyroid hormone production or increase the rate at which the body breaks down thyroid hormone. Lithium, used to treat bipolar disorder, is a well-known culprit—it blocks iodine uptake and hormone release, and can cause hypothyroidism in 20 to 30 percent of people who take it long-term. Amiodarone, a heart rhythm medication, contains high doses of iodine and can paradoxically cause hypothyroidism by overwhelming the thyroid's ability to regulate iodine use.

Other medications linked to hypothyroidism include some cancer immunotherapy drugs (checkpoint inhibitors), interferon-alpha (used for hepatitis C), and certain anticonvulsants. If you take any of these medications and develop symptoms of hypothyroidism—fatigue, weight gain, cold intolerance, dry skin—tell your doctor. Sometimes the medication can be switched; sometimes thyroid hormone replacement is added while continuing the necessary drug.

Pituitary and hypothalamic causes

The thyroid does not work independently. The pituitary gland, located at the base of the brain, produces thyroid-stimulating hormone (TSH), which signals the thyroid to produce T3 and T4. If the pituitary is damaged or does not function properly, it cannot send this signal, and thyroid hormone production drops even though the thyroid tissue itself is healthy. This is called secondary hypothyroidism or central hypothyroidism.

Pituitary damage can result from a pituitary tumor, surgery or radiation to the brain, head trauma, or conditions like pituitary apoplexy (sudden bleeding in the pituitary). The hypothalamus, a region above the pituitary, can also malfunction and fail to produce thyrotropin-releasing hormone (TRH), which triggers the pituitary to release TSH. Secondary hypothyroidism is less common than primary hypothyroidism (where the thyroid itself fails), but it requires different testing to diagnose because TSH levels alone can be misleading.

Other less common causes

Pregnancy can trigger postpartum thyroiditis, a temporary inflammation of the thyroid that causes hypothyroidism in the months after delivery. Most cases resolve within a year, though some women develop permanent hypothyroidism. Certain infections, including some viral infections, have been suspected of triggering thyroid problems, though the evidence remains incomplete.

Infiltrative diseases—conditions in which abnormal tissue invades the thyroid—are rare but can cause hypothyroidism. These include sarcoidosis, hemochromatosis (iron overload), and amyloidosis. Congenital hypothyroidism, present from birth, can result from thyroid agenesis (the thyroid fails to develop), dyshormonogenesis (the thyroid cannot make hormone properly), or TSH receptor defects. Newborn screening programs in most developed countries detect congenital hypothyroidism within days of birth.

Frequently Asked Questions

Can hypothyroidism go away on its own?

It depends on the cause. Postpartum thyroiditis often resolves within a year. Hypothyroidism caused by medication may improve if the drug is stopped. But Hashimoto's thyroiditis and hypothyroidism from thyroid surgery or radiation are permanent—the thyroid tissue damage cannot repair itself, and hormone replacement is needed for life.

Does stress cause an underactive thyroid?

Stress does not directly cause hypothyroidism, but it may worsen Hashimoto's thyroiditis in people genetically predisposed to it. Stress can also mask symptoms or make existing fatigue worse. If you have a family history of thyroid disease and experience prolonged stress, thyroid screening may be worth discussing with your doctor.

Can you develop hypothyroidism suddenly?

Most hypothyroidism develops gradually as thyroid tissue is slowly destroyed or damaged. However, thyroiditis (inflammation of the thyroid) can cause a sudden drop in hormone levels. Postpartum thyroiditis and silent thyroiditis can both appear abruptly. If you suddenly feel extremely fatigued, gain weight rapidly, or develop severe cold intolerance, contact your doctor for testing.

Is hypothyroidism hereditary?

Hashimoto's thyroiditis runs in families, so if a close relative has it, your risk is higher. However, having the genetic predisposition does not may provide you will develop the condition—environmental triggers also play a role. Family history is one reason doctors may screen for thyroid problems even without symptoms.

Can diet cause hypothyroidism?

Severe iodine deficiency from diet can cause hypothyroidism, but this is rare in countries with iodized salt. Goitrogens—compounds in foods like raw cabbage, broccoli, and soy—can interfere with iodine absorption in people already deficient in iodine, but they do not cause hypothyroidism in people with adequate iodine intake. A balanced diet with iodine-containing foods (dairy, eggs, seafood, iodized salt) prevents dietary deficiency.