Hyperthyroidism (overactive thyroid) doesn’t usually cause weight gain, though it may cause other weight changes. Sometimes, treatment for hyperthyroidism can lead to weight gain.

Hyperthyroidism (overactive thyroid) is a condition in which the body makes more thyroid hormone than it needs. This can speed up many of the body’s functions.

Thyroid hormone helps control how your body uses energy. A thyroid that produces too much hormone can speed up your metabolism.

Continue reading to learn how having an overactive thyroid may impact your weight, other causes of weight gain associated with hyperthyroidism, and other symptoms that hyperthyroidism can cause.

Thyroid hormone helps regulate your metabolism.

Your metabolism is how much energy your body uses and at what rate. This means that thyroid hormone also affects your basal metabolic rate. This rate is how much energy your body uses to function while at rest.

Too much thyroid hormone

Excess thyroid hormone is usually associated with a high basal metabolic rate. This means your body burns more energy while at rest, so weight loss is a common symptom of untreated hyperthyroidism. However, treatment for hyperthyroidism usually slows your metabolism, which can lead to weight gain.

Not enough thyroid hormone

Not producing enough thyroid hormone is usually associated with a low basal metabolic rate. Therefore, untreated hypothyroidism (underactive thyroid) can cause weight gain. Your body isn’t burning as much energy, which can lead to a calorie surplus.

Your metabolism is affected by a lot more than just thyroid hormone. Other hormones, diet, physical activity, and many other factors play a role. This means that thyroid hormone levels aren’t the whole story when it comes to losing or gaining weight from thyroid conditions.

Some people with hyperthyroidism may experience weight gain instead of weight loss. Some reasons why that might happen include:

Increased appetite

Hyperthyroidism usually increases appetite. If you eat a lot more calories, you can gain weight even if your body is burning more energy.

If you’re gaining weight due to increased appetite, you can try:

  • eating filling and nutrient-dense foods
  • getting regular exercise,
  • working with a health professional on a nutrition plan

These steps may help reduce unwanted weight gain from an increased appetite.

Hyperthyroidism treatment

Hyperthyroidism means your thyroid is producing too much thyroid hormone. Treatment brings your body back to producing a more normal amount. Because of this, even if you lose weight from untreated hyperthyroidism, you might gain weight after you start treatment. During treatment, your body starts making less thyroid hormone than before.

Some weight gain from treatment is usually expected, especially if you lost a lot of weight before treatment.

But if you’re concerned, talk with your doctor. You may need to readjust your calorie intake as your treatment takes effect. If the side effects of treatment, including weight gain, are intolerable to you, your doctor may adjust your treatment plan.

Thyroiditis

Thyroiditis is inflammation of the thyroid gland. It can cause either too high or too low levels of thyroid hormone. Hashimoto thyroiditis is an autoimmune condition that can cause hypothyroidism, which may be associated with weight gain.

Other symptoms of Hashimoto thyroiditis include:

If you start experiencing any of these symptoms, it’s best to talk with your doctor. They can help make a diagnosis and find the right treatment. Treatment for hypothyroidism due to Hashimoto thyroiditis generally involves thyroid hormone replacement medication.

When your body makes too much thyroid hormone, you may have other symptoms such as:

If you experience these symptoms, whether or not you’ve started treatment for hyperthyroidism, it’s best to let your doctor know.

Weight gain with hyperthyroidism can often be expected after treatment begins, especially if you previously lost a lot of weight due to your initially untreated condition.

However, if you’re gaining a lot of weight or have other uncomfortable symptoms, it might be worth bringing up with a doctor. Talk with a doctor about finding the right course of treatment for you.

It’s possible that treating hyperthyroidism can lead to hypothyroidism.

Weight gain alone is generally not an indicator of a thyroid issue. But weight gain alongside certain symptoms may indicate hypothyroidism:

  • fatigue
  • achiness
  • depression
  • dry skin
  • constipation

If you’re gaining weight and still have symptoms of hyperthyroidism, such as nervousness, increased sweating, and trouble sleeping, it’s also a good idea to talk with your doctor to find out the cause.

Weight gain with hyperthyroidism isn’t common, but it’s possible. It usually happens after you start treatment for hyperthyroidism and gain back the weight you previously lost when the condition was untreated.

In rare cases, it can mean something more serious. If you have hyperthyroidism and are gaining a lot of weight, talk with your doctor to figure out the best treatment or diet changes.