Overactive Thyroid: Does Hyperthyroidism Cause Weight Loss?

Hyperthyroidism is a condition caused by a malfunctioning thyroid gland. When your thyroid begins producing too large an amount of thyroid hormones, the entire body feels the adverse effect.

Since the thyroid gland controls the way you burn calories and produce energy, any impairment can impact your weight.  While it usually leads to weight loss, some people may gain extra pounds.

Let's take a closer look at how hyperthyroidism can cause weight loss and what you can do about it.

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What is hyperthyroidism?

Hyperthyroidism occurs when your thyroid gland begins producing too much of the hormone called thyroxine. Thyroxine (also known as T4) is responsible for many functions of your body including:

  • Digestion

  • Heart function

  • Muscle function

  • Brain development (in children)

  • Bone maintenance

T4 regulates the body’s metabolic rate, which is the rate at which your body burns calories and produces energy.

When the thyroid gland overacts and releases too much thyroxine, the body feels the effects of thyrotoxicosis. The short-term effects of this condition include weight loss, irregular heart rate, hair loss, irregular menstrual cycle, insomnia, goiter (enlarged thyroid), and more.

The causes of hyperthyroidism include: 

  • Graves' disease: This autoimmune disorder causes your immune system to produce antibodies that stimulate the thyroid gland to release excess thyroxine. Graves' disease is responsible for about 4 out of 5 hyperthyroidism cases. If Graves' disease runs in the family, you have a higher risk of developing the condition. 

  • Thyroid nodules: A benign lump of cells on your thyroid gland can start producing excess T4 hormones. While usually non-cancerous, thyroid nodules could cause the gland to swell and release hormones, causing hyperthyroidism.

  • Thyroiditis: This inflammation of the thyroid gland can cause excess hormones to leak into your bloodstream. Sometimes it happens due to autoimmune diseases. In some cases, it occurs after pregnancy. When the inflammation subsides, the thyroid gland can continue producing excess hormones.

  • Excess iodine consumption: Your thyroid gland uses iodine to produce thyroid hormones. If you are consuming too much iodine through your diet or via medication, you could develop hyperthyroidism.1 A healthy person can maintain normal thyroid function even after consuming excess iodine. However, patients who have previously suffered from episodes of thyroid problems, such as postpartum thyroiditis or thyrotoxicosis, may develop hyperthyroidism. 

The common symptoms of hyperthyroidism are:

  • Increased appetite

  • Weight loss

  • Rapid irregular heartbeat

  • Nervousness and mood swings

  • Insomnia

  • Fatigue

  • Irritability

  • Muscle weakness

  • Vision changes

  • Sweating and sensitivity to heat

  • Tremor 

  • Goiter (swelling in the neck)

Long-term effects of untreated hyperthyroidism include decreased bone mineral density, stroke, and heart failure.

In the US, 1% of people 12 years of age and older have hyperthyroidism. It's more common in women and people over 60.

With timely diagnosis and proper treatment, it's possible to keep the condition under control and improve your quality of life.

Hyperthyroidism and weight loss

Thyroid hormones regulate your metabolism. When the thyroid gland begins producing excess hormones, your metabolic rate increases. This means your body starts burning more calories than it needs. This can lead to weight loss.

Metabolism is measured by the amount of oxygen your body uses over a certain period. When this measurement is taken at rest, it's called the ‘basal metabolic rate’ (BMR). Your BMR shows how many calories your body uses up to maintain its normal functions, such as breathing, nutrient processing, blood circulation, and cell production.

People with an overactive thyroid gland usually have a higher BMR than those with a healthy thyroid. Doctors have been using BMR to diagnose hyperthyroidism for some time, but this method has been proven less effective than blood tests.

When your body produces more thyroxine than necessary, it speeds up the metabolic rate and burns more calories than it needs to survive. To maintain a healthy weight, you would need to consume many more calories than you are used to.

Does hyperthyroidism always cause weight loss?

Does hyperthyroidism-related weight loss mean that an overactive thyroid is a lifesaver for people who suffer from obesity? Not at all. Hyperthyroidism comes with many adverse consequences that could lead to serious outcomes.

Not all people who have hyperthyroidism lose weight. Many other hormones besides T4 can play a role in a person's metabolism. That's why the effect of a hyperactive thyroid gland on a person's body weight isn't always predictable.

Hyperthyroidism and weight gain

About 10% of people who have an overactive thyroid experience weight gain. The common reasons are:

Increased appetite

An increased appetite is a side effect of the excessive calorie-burning process. If a person starts consuming many more calories than before, the rapid metabolism caused by hyperthyroidism may not be enough to cause weight loss. 

It may even be possible to gain weight. Additionally, excessive food consumption could lead to a variety of other adverse side effects, including problems with your digestive system, such as:

  • Heartburn

  • Bloating

  • Gas


When you start anti-thyroid medications to treat your hyperthyroidism, hormone production returns to normal and you may start gaining weight. If your increased appetite due to untreated hyperthyroidism has caused you to habitually increase your food consumption, it can be hard to revert to your usual eating habits after the condition is treated. 

Many people start gaining weight when they begin treatment. Studies show that 38% of people experience a weight gain of 10% or more during hyperthyroidism treatment. To avoid weight problems, it's imperative to adjust your diet accordingly.

If you have surgery to remove your thyroid gland, you may develop hypothyroidism. Weight gain is one of the symptoms of this condition.

To control hypothyroidism, your doctor will prescribe hormone-replacement medication. It can take several weeks or even months to find the right dosage. During this time, your weight could fluctuate.

When to see a doctor

Unexplained weight loss is always a reason to see a doctor. If you haven’t made any drastic changes to your diet or workout routine, the weight loss may point to a thyroid problem.

Weight loss could be the first symptom of hyperthyroidism. Even if you aren’t concerned about your weight loss, if it is happening for no apparent reason, it's strongly advisable to speak to a doctor.

Consult your primary care physician who can organize preliminary tests to confirm your diagnosis. You might be referred to an endocrinologist for further management.

How hyperthyroidism is diagnosed

To diagnose hyperthyroidism, your doctor will:

  • Check your medical history: In addition to evaluating risk factors for hyperthyroidism, the doctor may ask about your family’s medical history and the timeline of your symptoms.

  • Perform a physical exam: A doctor will perform a thorough examination, including palpating your thyroid gland to check if it's enlarged, and measuring your pulse.

  • Order blood tests: The doctor will test your blood for levels of thyroid-stimulating hormone (TSH), T4, and thyroid antibodies. If T4 is high while TSH is low, you have an overactive thyroid. Other combinations could point to other conditions.

Additionally, a doctor could order a radioiodine scan for thyroid nodules and to see how iodine collects in your thyroid.

How hyperthyroidism is treated

Once your doctor makes a diagnosis, they will suggest a course of treatment. The treatment can include:

  • Anti-thyroid medication: This medicine prevents the thyroid gland from producing new thyroid hormones. While these meds can keep the disease under control for some patients, others could experience a relapse. 

  • Beta-blockers: While this medication doesn't treat hyperthyroidism, it can alleviate some symptoms, including irregular heartbeat and tremors. Doctors usually prescribe beta-blockers to complement other treatments.

  • Radioactive iodine therapy: This oral medication works by damaging the overactive thyroid cells. Usually, this treatment alleviates symptoms but leads to the destruction of the thyroid gland. Eventually, patients might start developing hypothyroidism.

  • Surgery: An operation to remove the thyroid gland may solve the hyperthyroidism problem for patients who did not respond to other treatments or if their enlarged thyroid gland is causing problems with breathing or swallowing. Thyroid gland removal (thyroidectomy) can cause hypothyroidism, which can be managed with hormone-replacement therapy (HRT).

Once the hypothyroidism treatment starts working, the symptoms begin to subside. If you were losing weight before the treatment began, you should start gaining it back. 

If you gain too much weight during your hypothyroidism therapy, speak to your doctor about it. It could mean that your treatment needs some adjustments.

The lowdown

Hyperthyroidism occurs when your thyroid begins to produce excess thyroid hormones. Your metabolism speeds up and, in most cases, causes weight loss. With the right treatment, it's possible to control the symptoms of hyperthyroidism and bring your weight up to normal levels.  

Since thyroid hormones aren't the only ones controlling your metabolism, weight loss might not be solely caused by an overactive thyroid. If you experience any unexplained weight fluctuations, it’s best to seek medical advice.

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