Hypothyroidism refers to an underactive thyroid, which can cause weight gain. Taking medication for this condition may help a person lose excess weight.

Treating an underactive thyroid may cause a person to lose some weight, but thyroid medication is not a weight loss drug. People who do not have an underactive thyroid should not take thyroid medications in an attempt to lose weight.

According to the American Thyroid Association (ATA), thyroid dysfunction can add 5–10 pounds (lb) of weight.

While thyroid medication may help people manage their condition, it is unlikely to dramatically change a person’s weight. In people without thyroid disease, taking thyroid hormones can be dangerous. It may cause an overactive thyroid, called hyperthyroidism.

Read on to learn about thyroid medications for weight loss, the benefits, side effects, and more.

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Thyroid hormone helps regulate many important functions, including metabolism.

A slower metabolism may cause a person to gain weight. People with hypothyroidism, or an underactive thyroid, may gain some weight. This is usually around 5–10 lb, but it may be more or less, depending on the individual.

Treating hypothyroidism may help reverse or stop weight gain. The main treatment for hypothyroidism is a synthetic thyroid hormone called levothyroxine.

Taking too much thyroid hormone may cause hyperthyroidism, a condition where the metabolism is too fast. Hyperthyroidism is dangerous and may even cause thyroid storm — a life threatening condition.

For this reason, it is not safe to use thyroid medication as a weight loss drug, take more than the amount a doctor prescribes, or take thyroid supplements without medical oversight. People should not take thyroid medication in an attempt to lose weight or without the guidance of a doctor.

Learn more about common thyroid disorders here.

The treatment for an underactive thyroid is replacing the missing thyroid hormone with synthetic hormones.

Doctors usually prescribe levothyroxine, an oral synthetic thyroid hormone.

This drug has many different brand names, including:

The Food and Drug Administration (FDA) has also approved an IV form of thyroid hormone for people who cannot take oral medication.

The FDA has also approved two other drugs, Armor Thyroid and Nature Thyroid. These drugs use a thyroid hormone made from animal hormones. They have similar side effects to other thyroid drugs.

Some people use these “natural” animal-based thyroid supplements as weight loss drugs. However, this is just as dangerous as using synthetic hormones for weight loss, and it can cause serious risks and side effects.

To improve the body’s ability to absorb synthetic thyroid hormone, people need to take it first thing in the morning, 30–60 minutes before a meal. A person should not take it with food.

Iron and calcium supplements and medications such as antacids may make it more difficult for the body to absorb thyroid medication. Individuals should wait to take these drugs until at least 4 hours after taking levothyroxine or other thyroid hormones.

Thyroid hormone is a treatment for hypothyroidism. Its main benefit is treating the condition by replacing the missing thyroid hormone.

This may improve overall health by reducing the risk of:

It may also improve symptoms such as:

Thyroid medication does not offer any specific health benefit to people without hypothyroidism. In fact, it may be harmful. It is not a “diet pill,” and it is unlikely to cause significant weight loss whether or not a person has hypothyroidism.

The main risk of levothyroxine is that too much of the drug can cause hyperthyroidism and, potentially, a dangerous condition called thyroid storm.

In older people and those with underlying heart disease, too much thyroid hormone can cause serious heart-health issues. These may include arrhythmias and heart damage.

Some potential side effects, especially for those who take too much thyroid medication, include:

People taking thyroid medication who develop these side effects should contact a doctor. They can assess their symptoms and test their thyroid stimulating hormone (TSH) levels.

A person may be taking too much thyroid medication. In this case, a doctor can lower their recommended dose.

There is no proven non-drug treatment for hypothyroidism.

Talking with a doctor before trying alternative therapies is important, especially if a person has severe hypothyroidism. Using an alternative therapy instead of a proven treatment can be dangerous.

People with hypothyroidism who are hoping to lose weight may find additional support from an endocrinologist, who can evaluate lifestyle, diet, exercise, and factors such as insulin resistance and overall health.

Thyroid medication is not a weight loss drug, and people should not use it as such. For this reason, there is no best thyroid medication for weight loss in a person who does not have hypothyroidism.

Anyone concerned about their weight should contact a doctor and ask about testing for hypothyroidism and other conditions, such as diabetes. People should never take any prescription drug without a doctor’s permission.