Hyperthyroidism and Thyrotoxicosis: Symptoms, Causes, Treatments

Hyperthyroidism is a condition in which the thyroid glands produce too much thyroid hormone. It affects around 1% to 3% of the US  population,¹ increases with age, and is more prevalent in women.

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What is hyperthyroidism?

Hyperthyroidism, also referred to as thyrotoxicosis, is a condition where you have an overactive thyroid. This means your thyroid gland produces an excess of thyroid hormones.

Your thyroid is a butterfly-shaped gland located at the front of your neck which plays a significant role in many bodily functions, including:

  • Controlling your heart rate

  • Regulating your body temperature

  • Controlling your metabolism

When your thyroid gland functions properly, it keeps your body in balance and your systems functioning well. However, if the thyroid stops working correctly, it creates too few or too many thyroid hormones, which can negatively affect your entire body.

You have a higher chance of experiencing hyperthyroidism if you have a family history of thyroid disease or if you have another health problem,² such as:

  • Primary adrenal insufficiency (a hormone disorder)

  • Type 1 or type 2 diabetes

  • Pernicious anemia (a disorder due to a deficiency in vitamin B12)

Hyperthyroidism can also occur if you:


The thyroid hormone helps to regulate your various bodily processes, referred to as your metabolism. If you have too much thyroid hormone, it causes your bodily functions to speed up.

Symptoms of hyperthyroidism include:

  • Irritability

  • Nervousness

  • Heart racing

  • Increased sweating

  • Trouble sleeping

  • Anxiety

  • Hand tremors

  • Fine, brittle hair

  • Thinning of your skin

  • Muscle weakness, particularly in your thighs and upper arms

  • More frequent bowel movements, but diarrhea isn't common

If you have hyperthyroidism, you may lose weight even if you feel you are eating enough. Many women with hyperthyroidism may experience a lighter and less frequent menstrual flow. Because hyperthyroidism raises your metabolism, you may have a lot of energy at first, but as the condition persists, your body will likely start breaking down, causing you to feel tired.

Hyperthyroidism typically starts slowly, although the changes can be sudden in younger individuals. Initially, you may mistake the symptoms for stress and anxiety. If you've been dieting to lose weight, you may be happy with the weight loss that comes with hyperthyroidism until it starts causing other problems.

If you have the most common type of hyperthyroidism, Graves' Disease, your eyes may appear enlarged due to your upper lids being elevated. In some cases, one or both of your eyes may bulge. Some people experience swelling of the front of their neck because of a goiter (enlarged thyroid gland).

Hyperthyroidism can lead to serious health issues if left untreated, including²:

  • An eye disorder known as Graves' ophthalmopathy

  • An abnormal heartbeat that can cause stroke, heart failure, blood clots, and other heart-related issues

  • Fertility and menstrual cycle problems

  • Muscle problems, osteoporosis, and thinning bones

Relationship between thyroid function and weight gain

According to the American Thyroid Association,³ there's a complex relationship between metabolism, body weight, and thyroid disease. The thyroid hormone regulates metabolism, and your metabolism is determined by calculating how much oxygen your body uses over a certain period.

The calculation while resting is referred to as your basal metabolic rate (BMR). BMR measurement is one of the initial tests used to evaluate a person’s thyroid status. Those with an underactive thyroid gland have a low BMR, whereas an overactive thyroid gland results in a high BMR.

Does hyperthyroidism cause weight gain?

Some research shows that hyperthyroidism leads to weight loss in most people, but it has variable effects, and some people gain weight.⁴ Because hyperthyroidism can also increase your appetite, some people experience weight gain, depending on how many more calories they're taking in.

Hyperthyroidism treatment can also cause weight gain. Since hyperthyroidism is an irregular state, any weight you lose will come back once your body returns to its normal state. Typically, once hyperthyroidism is treated, any hyperthyroid-related weight loss is regained. 

Every person’s weight change will be different, and different treatments have varying impacts on body weight. Some research indicates that radioiodine causes more weight increase (or even surgery) than anti-thyroid medications. 


There is a range of different treatments for hyperthyroidism, including:

1. Antithyroid medication

Two types of medication are available to treat hyperthyroidism:

  • Methimazole (Tapazole)

  • Propylthiouracil (PTU)

Usually, methimazole is preferred since PTU can lead to fatal liver damage, although this is a rare side effect. These medications slow the production of the thyroid hormone to control hyperthyroidism. It may take a few months to bring your thyroid hormone levels back to normal.

After treatment with antithyroid medication, some people go into remission of hyperthyroidism, but this is usually temporary.

Antithyroid medication can result in side effects such as allergic reactions, like hives or a rash. Also, in rare situations, the drugs may increase your susceptibility to infection.

2. Radioactive iodine treatment

This is typically used to treat hyperthyroidism cases resulting from thyroid hormone overproduction. You take radioactive iodine orally, which enters your bloodstream into your thyroid cells and slowly destroys them. It could take a few months before you start to see results.

Since radioiodine eventually destroys all thyroid cells, it's impossible to eliminate an exact amount of the damaged thyroid gland. So, most endocrinologists aim to destroy the damaged thyroid gland with just one dose of radioiodine. This results in hypothyroidism (underactive thyroid), a condition that is the opposite of hyperthyroidism, which is corrected by taking thyroid hormone replacement therapy for life.

3. Surgical thyroid removal

In rare cases, your doctor may recommend surgery to remove most of your thyroid gland. For example, if you're a pregnant woman with severe symptoms and radioiodine wouldn't be safe for your baby, you may be a candidate for surgical thyroid removal.

Surgery typically causes hypothyroidism, which you can then treat with lifelong thyroid hormone replacement therapy.

4. Other types of treatments

Beta-blockers can be used alongside another form of treatment. They temporarily control symptoms of hyperthyroidism until the other treatment takes effect. If your hyperthyroidism is due to excessive ingestion of either thyroid hormone or iodine or caused by the condition thyroiditis, this may be the only treatment you need.

Your doctor will prescribe iodine drops before your surgery if you have Graves' disease or if your hyperthyroidism is severe.

Following treatment, you'll likely require further treatment in the form of lifelong replacement thyroid hormone. This is because certain treatments, particularly surgery, cause your thyroid hormone levels to become extremely low or eliminate the thyroid hormone altogether by removing your thyroid. Your doctor will need to reintroduce thyroid hormones back into your body by prescribing regular medication.

There is always the risk of side effects with any treatment. It's important that you consult with your doctor and weigh up all of the risks and benefits before deciding on a treatment plan under the guidance of your doctor.

When to speak to a healthcare professional 

See your doctor if you experience any hyperthyroidism-related symptoms, such as:

  • Unusual sweating

  • Rapid heartbeat

  • Swelling at the base of your neck

  • Unexplained weight loss

  • Any other symptoms linked with the condition

Make sure to discuss any side effects with your doctor and describe them in detail, as many of the symptoms or signs of hyperthyroidism are shared by various other health conditions.

If you have already received a hyperthyroidism diagnosis, you'll likely have to see your doctor regularly on an ongoing basis to monitor your treatment and make sure it is effective.

If you experience any symptoms of thyroid storm (a hyperthyroidism complication), such as an extremely rapid heart rate or high fever, seek immediate medical attention at your nearest hospital.

The lowdown

Hyperthyroidism is a treatable and manageable condition involving the overproduction of the thyroid hormone by the thyroid gland. Most individuals respond well to treatment, but certain types of treatment will require you to take lifelong medication to normalize your thyroid levels in the long term.

While hyperthyroidism typically causes weight loss, treatment for this condition can cause weight gain.

Hyperthyroidism caused by Graves' disease that's left untreated may worsen over time, causing serious complications.

If you are experiencing hyperthyroidism and/or have Graves’ disease, make sure to consult with your doctor about the right treatment plan for you.

  1. Hyperthyroidism (2016)

  2. Hyperthyroidism (Overactive thyroid) | National Institute of Diabetes and Digestive and Kidney Diseases

  3. Thyroid and weight | American Thyroid Association

  4. Treatment for hyperthyroidism increases the risk of obesity in some patients | American Thyroid Association

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