What Effect Does Thyroid Hormone Have on the Body's Metabolic Rate?

Thyroid hormones help regulate the body's metabolic rate, which is a measure of how efficient it is at using energy. When the thyroid isn’t producing enough of certain hormones, metabolism slows down; when levels of these hormones are too high, metabolism increases.

However, this doesn't mean that low levels of thyroid hormone mean a person will gain weight (or conversely that high levels will ignite weight loss.) There are many factors involved in these processes and the relationship between thyroid hormones, metabolic rate, and weight is not entirely understood.

Metabolism

Metabolism involves a series of processes, including breaking down food and converting it into energy, and determining how quickly (or slowly) the body uses that energy. The efficiency at which it does this is called the basal metabolic rate (BMR).

While the thyroid doesn't cause a fast or slow metabolism, it does play a major role in controlling the body's metabolism. Thyroid hormones work along with many other hormones, proteins, nuclear receptors, and chemicals to help maintain a person's BMR.

Thyroid Hormones

The thyroid is controlled by the brain's pituitary gland. This gland produces thyroid-stimulating hormone (TSH), which signals the thyroid to produce hormones.

The two main hormones produced by the thyroid include:

• Triiodothyronine (T3): This hormone helps the body maintain control of muscles, as well as many organ functions—including those of the heart, brain, and digestive system. It also plays a role in bone health and the body's metabolic rate.
• Thyroxine (T4): This hormone increases the speed of chemical reactions within cells and is an integral part of cell growth and development. A synthetic version of thyroxine is used to treat thyroid disorders.
  

Weight

A person's body burns calories (energy) during physical activity and also during rest. When there is excess energy—the result of eating more food than the body can use—this energy gets stored in fat cells and weight increases.

Research on the relationship between thyroid hormones and weight gain has returned slightly different conclusions. However, most studies have found that in people with obesity, levels of T3 are generally high.

Thyroid Diseases

Thyroid diseases impact the gland's production of hormones, which in turn can affect a person's metabolism. These conditions include:

• Hypothyroidism: This is a disease where a person's thyroid doesn't produce adequate amounts of thyroid hormones and it is associated with an increase of 5 to 10 pounds in body weight. Typically, the more severe the hypothyroidism, the greater a person's weight gain. What's more, up to 60% of adults with obesity have an autoimmune condition that results in hypothyroidism.
• Hyperthyroidism: Unexplained weight loss is often a symptom of hyperthyroidism, a condition where the thyroid is overactive and produces more hormones than needed. Many people with an overactive thyroid lose weight; in general, the more severe the hyperthyroidism, the more weight that is lost.

Hormone Treatment and Weight

Hypothyroidism is typically treated with a synthetic version of the thyroid hormone thyroxine. Treatments for hyperthyroidism include medication, radioactive iodine therapy, and surgery.

Treatment usually returns body weight to what it was before the disease. That means a person may lose weight when their hypothyroidism is under control or gain weight after hyperthyroidism treatment.

If you're concerned about your weight, talk to your healthcare provider. They can suggest safe options to help you manage your weight.

Summary

The relationship between thyroid hormones and metabolic rate is complex. However, in general, high levels of certain thyroid hormones increase a person's metabolic rate and low levels decrease it.

This likely explains why people with hyperthyroidism tend to have unexplained weight loss while those with hypothyroidism often gain weight. Metabolism and weight usually return to normal once a person's thyroid disease is under control.