Hyperthyroidism happens when your thyroid gland produces more hormone than your body needs, speeding up your metabolism. The most common cause is Graves’ disease, an autoimmune condition responsible for 60% to 80% of all cases. But several other conditions, medications, and even dietary supplements can trigger it too.
Women are about three times more likely than men to develop hyperthyroidism, and prevalence rises with age, peaking in women between ages 50 and 54 before climbing again after 75.
Graves’ Disease
Graves’ disease is by far the leading cause. It’s an autoimmune condition in which your immune system produces antibodies that mimic the hormone your brain normally sends to tell your thyroid how much hormone to make. These antibodies latch onto the same receptors on thyroid cells and essentially press the gas pedal without any brake. The thyroid responds by ramping up hormone production and physically growing larger, which is why people with Graves’ disease often develop a visibly swollen neck (goiter).
Genetics play a major role. Research estimates that genetic predisposition accounts for roughly 79% of the risk for Graves’ disease, with environmental factors making up the remaining 21%. About 70% of the genes linked to autoimmune thyroid disorders are involved in immune cell function, which helps explain why the condition runs in families. Environmental triggers include smoking, excess iodine intake, selenium and vitamin D deficiency, and certain occupational chemical exposures.
Thyroid Nodules That Overproduce Hormones
Sometimes one or more lumps (nodules) on the thyroid start producing hormones on their own, ignoring the normal signals from the brain. When a single nodule does this, it’s called a toxic adenoma. When multiple nodules are involved, the condition is called toxic multinodular goiter. In a multinodular goiter, not every nodule is necessarily overactive. Some may sit quietly while others churn out excess hormone.
These nodular conditions are more common in older adults and in people who have had an enlarged thyroid for years. Unlike Graves’ disease, the overproduction here isn’t driven by the immune system. Instead, the nodule tissue has essentially gone rogue, functioning independently of the body’s feedback loop.
Thyroid Inflammation (Thyroiditis)
Inflammation of the thyroid gland can cause stored hormone to leak into your bloodstream all at once, creating a temporary surge. This isn’t your thyroid making too much hormone. It’s releasing what was already there, like a cracked container spilling its contents.
Three common forms follow a similar pattern. Subacute thyroiditis typically follows a viral infection and causes neck pain and tenderness. Silent thyroiditis is painless and often linked to an underlying autoimmune tendency. Postpartum thyroiditis affects some women in the months after giving birth. All three tend to follow a predictable three-phase course: a hyperthyroid phase lasting one to three months, a hypothyroid phase (too little hormone) lasting up to six months, and then a gradual return to normal. Because the overactive phase is short-lived, treatment focuses on managing symptoms rather than shutting down the thyroid.
Too Much Iodine
Your thyroid needs iodine to make hormones, but flooding it with too much iodine can backfire. In people with a healthy thyroid, excess iodine temporarily slows hormone production as a safety mechanism. But in people who already have underlying thyroid problems, like nodular goiters or early autoimmune disease, this safety brake can fail. Instead of slowing down, the thyroid uses the extra iodine to overproduce hormones.
Common sources of excess iodine include contrast dyes used in CT scans and other imaging procedures, iodine-based antiseptic solutions, and dietary supplements. Iodine supplements are widely available and increasingly popular, which means the risk isn’t limited to medical settings. If you have a history of thyroid nodules or goiter, even an over-the-counter multivitamin with high iodine content could be a trigger.
Medications
Certain prescription drugs can push the thyroid into overdrive. The best-studied example is amiodarone, a heart rhythm medication. Each standard tablet contains a substantial amount of iodine, and about 3% of patients taking it in iodine-sufficient regions develop hyperthyroidism. In areas where diets are naturally low in iodine, that number jumps to around 10%.
Amiodarone causes thyroid problems in two distinct ways. In patients who already have an underlying thyroid condition, the drug’s iodine load overwhelms the gland and accelerates hormone production. In patients with a previously normal thyroid, the drug’s chemical structure can directly damage thyroid cells, causing them to release stored hormones into the bloodstream, similar to what happens during thyroiditis. Higher doses carry greater risk.
Pregnancy-Related Hyperthyroidism
During the first trimester of pregnancy, levels of the hormone hCG rise sharply. HCG has a molecular structure similar enough to thyroid-stimulating hormone that at very high levels, it can nudge the thyroid into producing extra hormones. This is particularly common in women with severe morning sickness (hyperemesis gravidarum), where hCG levels tend to be especially elevated.
This type of hyperthyroidism is transient. It resolves on its own as hCG levels naturally fall in the second trimester. It’s important to distinguish it from Graves’ disease, which can also appear or flare during pregnancy but requires different management. The distinction is made through blood tests that check for the specific antibodies found in Graves’ disease.
Pituitary Tumors
In rare cases, the problem isn’t the thyroid itself but the pituitary gland at the base of the brain. A noncancerous pituitary tumor can continuously secrete the signal hormone that tells the thyroid to produce more. The hallmark of this condition is a hormonal pattern that looks “wrong” in the usual way: thyroid hormone levels are high, but the pituitary signal that should be suppressed in response remains normal or elevated instead.
Because these tumors grow inside the skull, they can also cause headaches and vision problems as they press on nearby structures. Some pituitary tumors produce additional hormones beyond the thyroid-stimulating one, leading to other symptoms like abnormal growth or unexpected breast milk production. These tumors account for a very small fraction of hyperthyroidism cases, but they require different treatment since the root cause is in the brain rather than the thyroid.
How These Causes Are Told Apart
Blood tests are the starting point. They measure levels of thyroid hormones and the pituitary signal that controls them. In most forms of hyperthyroidism, thyroid hormones are high while the pituitary signal drops to near zero, because the brain recognizes there’s too much hormone and stops asking for more. When both are elevated, that points to a pituitary tumor or a rare genetic condition affecting hormone sensitivity.
From there, the specific cause is narrowed down through antibody testing (positive in Graves’ disease), imaging of the thyroid to look for nodules or inflammation patterns, and clinical context like recent pregnancy, medication history, or iodine exposure. The cause matters because treatment varies significantly. Graves’ disease and toxic nodules often require long-term intervention, while thyroiditis and pregnancy-related hyperthyroidism typically resolve on their own.