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

The thyroid gland is in the neck. It has a butterfly shape. The hormones that it produces and releases into the bloodstream control the body's growth and metabolism. Too much hormone can affect many functions in the body.

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

Hyperthyroidism is distinct from hypothyroidism. "Hyper" refers to the presence of too much thyroid hormone in the system. "Hypo" means too little, or an underactive thyroid.

Around 1.2 percent of people in the United States (U.S.) have an overactive thyroid.

It affects women more than men and is most likely to occur over the age of 60 years.

Without treatment, hyperthyroidism can seriously impact different bodily functions, including the heart. However, medication can normally control it by reducing thyroid hormone production.

Fast facts on hyperthyroidism

Here are some key points about hyperthyroidism. More detail is in the main article.

  • Hyperthyroidism happens when the thyroid gland produces too much hormone.
  • Graves' disease is the most common cause.
  • Medication can usually normalize hormone levels, but treatment may take 1 to 2 years.
  • Untreated, hyperthyroidism can cause severe complications.
  • With treatment, pregnancy should be able to progress normally.

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The thyroid gland is vital for regulating hormones around the body. When the thyroid produces too many hormones, this is known as hyperthyroidism.

Symptoms are wide ranging, and they vary between people. Patients with mild hyperthyroidism are often unaware that they have it because there are no symptoms.

Most symptoms are related to the increase in metabolic rate.

They include:

  • swelling in the neck caused by an enlarged thyroid gland, or goiter
  • nervousness, irritability, mood swings, and decreased concentration
  • diarrhea
  • difficulty breathing
  • fatigue, tiredness, 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 (urticaria)
  • nails become loose
  • menstrual problems in women, especially lighter periods or absence of periods
  • alopecia where hair is lost in patches
  • accelerated heartbeat, sometimes with palpitations
  • redness on the palms of hands
  • sudden weight loss or gain
  • trembling hands and shakiness

Symptoms vary between individuals, and patients rarely have all these symptoms.

Patients 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 risks.

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

Beta-blockers can relieve symptoms until other treatments start to work. According to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), most people will feel better within a few hours.

Antithyroid drugs stop the thyroid gland from producing too much thyroxine or triiodothyronine. Methimazole or propylthiouracil (PTU) may be used.

After starting treatment, it may take several weeks or months for hormone levels to adjust to the normal range. On average, total treatment time is between 1 and 2 years, but it can take longer.

Adverse effects of medications include:

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

Radioactive iodine is picked up by the active cells in the thyroid, and it 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 for women who are pregnant or breastfeeding. Women should avoid getting pregnant for at least 6 months after treatment, and men should not father a child for at least 4 months.

Surgery can remove part of the thyroid gland if other treatments are not possible, for instance, during pregnancy, cannot tolerate other therapies, or has cancer.

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

Several factors can cause hyperthyroidism.

Graves' disease

This is the most common cause, and it accounts for over 70 percent of cases. It is an autoimmune condition.

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

Grave's disease is most common in women aged 20 to 40 years and especially smokers.

It can affect the eyes, causing discomfort and double vision. Patients commonly have eyes that protrude.

Nodular thyroid disease

Lumps, known as nodules, develop in the thyroid gland. It is unclear why they develop. They 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 there is no pain. Someone with nodules may feel them 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 (T4) and triiodothyronine (T3).

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

Intake of thyroid hormones: Patients who are receiving thyroid hormones as a treatment need to follow up regularly with their doctor to make sure they are taking the right dose of thyroid medication.

Medications

Some medications used in the treatment of heart problems contain a large amount of iodine. They may trigger changes in thyroid function. Options include amiodarone and lithium, used to treat bipolar disorder.

Thyroiditis

An inflammation of the thyroid that often results from a viral infection. Symptoms include fever, sore throat, painful swallowing, generalized aches, and pains in the neck.

Follicular thyroid cancer

In rare cases, 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 stop thyroid activity becoming more imbalanced.

A balanced diet can help to preserve thyroid health. If you are choosing to take supplements, be sure to ask your doctor for advice on how much to take and which supplements will be best for you without affecting thyroid activity.

Graves' ophthalmopathy 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 is an uncommon reaction that can be triggered by an infection, injury, or trauma, including surgery, or by childbirth. It can also occur in pregnant women with undiagnosed or poorly controlled hyperthyroidism.

The signs and symptoms include rapid heartbeat, high fever, agitation, jaundice, vomiting, diarrhea, dehydration, and hallucinations.

This is a life-threatening reaction. It requires emergency medical treatment.

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 well patients follow their treatment plan.

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

Advanced hyperthyroidism is normally easy to diagnose because the signs are clear, but diagnosis is less obvious in the early stages.

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 (T4), and triiodothyronine (T3).

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 (RAIU) test.

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A women with an overactive thyroid who wants to become pregnant should seek treatment for their hyperthyroidism first.

A woman with hypothyroidism may find it harder to become pregnant.

During pregnancy, thyroid hormone levels will rise slightly.

Women with an overactive thyroid may find that their thyroid enlarges slightly during pregnancy. Some women who are susceptible but have not been diagnosed before may have a slightly hyperactive thyroid during pregnancy.

Severe, untreated hyperthyroidism during pregnancy has been linked to spontaneous abortion, low birth weight, maternal high blood pressure, and heart problems.

If the mother has a thyroid problem, the newborn should be checked for thyroid function, as these can have a profound effect on brain development.

Women who are receiving treatment before pregnancy will continue to receive the same therapy, but their medication may need adjusting as thyroxine dose requirements usually increase. Levothyroxine is safe to take during pregnancy, as it has the same characteristics as the natural hormone.

However, it should be taken 2 to 3 hours apart from any prenatal vitamins, because iron and calcium may interfere with the absorption of thyroid hormone.

A woman should ensure her hyperthyroidism is under control before becoming pregnant.

With appropriate treatment, most pregnancies can be expected to progress normally.