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 is how much energy your body uses to keep functioning while it’s at rest.

In most cases, excess thyroid hormone is associated with a high basal metabolic weight. This means that your body burns more energy while it’s at rest, so weight loss is a common symptom of hyperthyroidism.

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

But your metabolism is affected by a lot more than just thyroid hormone. Other hormones, how much and what you eat, your 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 might experience weight gain instead of the more common weight loss. Some reasons why that might happen include:

Increased appetite

Hyperthyroidism usually increases your appetite. If you’re taking in a lot more calories, you can gain weight even if your body is burning more energy. Make sure you eat healthy foods, get regular exercise, and work with a doctor on a nutrition plan. These steps can all help combat weight gain from an increased appetite.

Hyperthyroidism treatment

Hyperthyroidism is an abnormal state for your body. Treatment brings your body back to its normal state. Because of this, when you lose weight from hyperthyroidism, you might gain some weight back after you start treatment. Your body starts making less thyroid hormone than it was before.

Some weight gain from treatment is usually fine, especially if you lost a lot of weight before treatment. If you’re concerned, talk to 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 can help you find a new treatment.

Thyroiditis

Thyroiditis is an inflammation of the thyroid. This can cause either too high or too low levels of thyroid hormone. The most common type of thyroiditis is Hashimoto disease. It’s also the most common cause of hypothyroidism.

In some rare cases, the immune response to Graves disease — the most common type of hyperthyroidism — can continue long enough to attack the thyroid and lead to inflammation. Therefore, it can cause Hashimoto disease, which can in turn cause weight gain.

Other symptoms of Hashimoto disease are:

  • fatigue
  • dry skin
  • constipation
  • depression

If you start experiencing any of these symptoms, see your doctor. They can help make a correct diagnosis and find the right treatment for you. Treatment for Hashimoto disease is generally thyroid hormone replacement pills.

In most cases, weight gain with hyperthyroidism is probably nothing to worry about, 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 indicate a new problem. Talk to your doctor about finding the right course of treatment for you.

Weight gain alone is generally not a sign of a thyroid problem. But weight gain alongside the following symptoms can indicate hypothyroidism:

See a doctor if you develop any of these symptoms. If you’re gaining weight and have symptoms of hyperthyroidism, such as nervousness, increased sweating, and trouble sleeping, it’s still a good idea to see your doctor. They can help you find the right diagnosis and treatment.

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

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