Hyperthyroidism, or overactive thyroid, happens when the thyroid gland produces too much hormone. This has impacts throughout the body.

The thyroid is a butterfly shaped gland in the neck. The hormones that it produces and releases into the bloodstream control the body’s growth and metabolism.

There are numerous causes of hyperthyroidism and a wide range of possible symptoms. It usually begins slowly, but, in younger people, onset can be sudden.

Around 1.2% of people in the United States have an overactive thyroid. Hyperthyroidism affects more females than males and is most likely to occur in people over 60.

Hyperthyroidism is distinct from hypothyroidism, or an underactive thyroid. “Hyper” means there is too much thyroid hormone in the system, and “hypo” means there is too little.

Without treatment, hyperthyroidism can have severe complications. Medication can normally control it by reducing thyroid hormone production.

This article provides an overview of hyperthyroidism, including its causes, symptoms, diagnosis, complications, and treatments.

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Hyperthyroidism happens when the thyroid gland produces too much of hormones called thyroxine and triiodothyronine.

People with mild hyperthyroidism may have no symptoms and are often unaware that they have it.

If symptoms arise, they can affect the whole body and many body functions. Most symptoms are related to an increase in metabolism caused by excess thyroid hormone.

Symptoms vary between individuals and can include:

  • a goiter, which is swelling in the neck caused by an enlarged thyroid gland
  • nervousness, irritability, mood swings, and reduced concentration
  • diarrhea
  • difficulty breathing
  • fatigue and difficulty sleeping
  • muscle weakness
  • hyperactivity
  • oversensitivity to heat, excessive sweating, and warm, damp skin
  • increased appetite
  • increased bowel movements and urination
  • infertility and a loss of interest in sex
  • itchy skin with raised, itchy swellings, called hives or urticaria
  • nails becoming loose
  • menstrual problems, especially lighter periods or absence of periods
  • alopecia or patchy hair loss
  • a faster heartbeat, sometimes with palpitations
  • redness on the palms of hands
  • sudden weight loss or gain
  • trembling hands and shakiness

People with diabetes may experience heightened diabetes symptoms, such as fatigue and increased thirst.

Those with heart disease have a higher risk of arrhythmia, heart failure, and other cardiovascular problems.

Some medications treat the symptoms of hyperthyroidism, such as heart problems, while others target thyroid hormone productions.

Beta blockers

Beta-blockers cannot treat hyperthyroidism, but they can reduce the symptoms until other treatments take effect. People may feel the effects within a few hours.

Antithyroid drugs

Antithyroid drugs stop the thyroid gland from producing too much thyroid hormone.

The most often used drug is methimazole. During pregnancy, a doctor may recommend the drug propylthiouracil instead, as methimazole may harm a fetus.

According to the American Thyroid Association, “In about 20% to 30% of patients with Graves’ disease, treatment with antithyroid drugs for a period of 12 to 18 months will result in prolonged remission of the disease.”

Adverse effects of medications can include:

  • allergic reactions
  • reduction in white blood cells, increasing the chance of infections
  • rarely, liver failure

Radioactive iodine

Radioactive iodine is picked up by the active cells in the thyroid and then destroys them. The destruction is local, and there are no widespread side effects. The dose of radioactivity contained in the radioiodine is very low and is not harmful.

Radioiodine treatment is not suitable during pregnancy or breastfeeding. Doctors recommend avoiding pregnancy for 6–12 months after treatment.

Surgery

Surgery can remove part of the thyroid gland if other treatments are not possible. This group might include people who are pregnant, those who cannot tolerate other therapies, or those who have cancer.

Read more about thyroid gland removal here.

There is a range of possible causes of hyperthyroidism, including:

Graves’ disease

Graves’ disease is the most common cause of hyperthyroidism, accounting for over 70% of cases. It is an autoimmune condition.

It is unclear what triggers Graves’ disease, but it often runs in families, suggesting a genetic basis.

Graves’ disease is most common in women aged 30–50, and is seven to eight times more likely in females compared with males.

It can affect the eyes, causing protracted eyelids, bulging eyes, double vision, and swelling around the eyes.

Nodular thyroid disease

Thyroid nodules are lumps that develop in the thyroid gland. It is unclear why they develop.

These lumps may contain abnormal thyroid tissue, but they are usually benign, or noncancerous. They affect the regular function of the thyroid, causing overactive thyroid.

The thyroid may become enlarged, but people usually do not experience pain. The person may be able to feel the nodules with the fingertips.

Excessive iodine intake

The thyroid gland removes iodine from the blood. Iodine comes from foods such as seafood, bread, and salt. The thyroid gland uses the iodine to produce thyroid hormones.

The two most important thyroid hormones are thyroxine and triiodothyronine.

Taking additional iodine in supplements can cause the thyroid gland to produce too much of the hormones.

People who take thyroid hormones as medication should follow up regularly with their doctor to make sure they are taking the right doses.

Medications

Some medications that treat heart problems contain a large amount of iodine. They may trigger changes in thyroid function. Another drug that can affect thyroid function is lithium, which is a drug used to treat bipolar disorder.

Thyroiditis

Thyroiditis, an inflammation of the thyroid, often results from a viral infection. Symptoms include:

Follicular thyroid cancer

In rare cases, an overactive thyroid can be caused by thyroid cancer. The malignant cells may start producing thyroxine or triiodothyronine.

There is no special diet that can resolve a thyroid disorder.

However, reducing the intake of excessive iodine in the diet and avoiding iodine supplements can help to keep thyroid activity from becoming more imbalanced.

A balanced diet can help preserve thyroid health. If a person chooses to take supplements, they should ask the doctor for advice on how much to take and which supplements will not affect thyroid activity.

People may find it useful to avoid foods and other products that are high in iodine, such as seaweed and some cough medicines and multivitamins.

Read more about foods for hyperthyroidism here.

The severity of hyperthyroidism and its symptoms depend on how well the body is able to react to the changes that result from the excess thyroid hormones, and how closely a person is able to follow their treatment plan.

Graves’ ophthalmopathy

Graves’ opthalmopathy can cause pain or discomfort in the eye, light sensitivity, and some vision problems. The eyes may protrude.

Using eye drops and wearing sunglasses can help relieve symptoms.

In severe cases, treatment with certain drugs, such as steroids or immunosuppressive drugs, can decrease the swelling behind the eyes.

A thyroid storm

A thyroid storm is an uncommon reaction that can occur after an infection, injury, or physical trauma, such as surgery or childbirth. It can also occur in pregnancy if the person has undiagnosed or poorly controlled hyperthyroidism.

This is a life threatening reaction and it requires emergency medical treatment.

The signs and symptoms include:

To diagnose hyperthyroidism, a doctor will ask about symptoms, perform a physical examination, and possibly order blood tests.

Diagnosing advanced hyperthyroidism is normally straightforward because the signs are clear, but in the early stages the signs are less obvious.

A blood test, known as a thyroid function test, can show how well the thyroid gland is working. The test checks for levels of thyroid-stimulating hormone (TSH), thyroxine, and triiodothyronine.

Learn more about the TSH test here.

A special diagnostic scan of the thyroid gland may be carried out using radioactive iodine to gauge thyroid function. This is known as a radioactive iodine uptake test.

Females with hyperthyroidism may have more difficulty becoming pregnant.

Thyroid hormone levels rise slightly during pregnancy. Some females who are susceptible but have not received a diagnosis may have a slightly hyperactive thyroid during pregnancy.

Those with an overactive thyroid may find that their thyroid enlarges slightly during pregnancy.

Severe, untreated hyperthyroidism during pregnancy has been linked to:

If the mother has a thyroid problem, the doctors should check the newborn’s thyroid function, as thyroid problems can have a significant effect on brain development.

People who are receiving treatment before pregnancy will continue to receive the same therapy, but a doctor may adjust their medication as they may need higher doses of thyroxine than before.

Levothyroxine is safe to take during pregnancy, as it has the same characteristics as the natural hormone.

With appropriate treatment, most pregnancies progress normally.

Hyperthyroidism, or an overactive thyroid, is more common among females. It has various causes, the most common being Graves’ disease.

People can treat hyperthyroidism and manage its symptoms using various medications. Some diet changes may also help.