Hyperthyroidism is a complex condition that has wide-ranging effects on the body. The disease affects the entire body and may lead to severe health complications. While the condition usually leads to weight loss, you can have hyperthyroidism and still gain weight.
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Hyperthyroidism, also known as an overactive thyroid, is a condition that occurs when your thyroid gland produces more hormones than your body needs. Your thyroid plays a role in virtually every function of your body and significantly impacts your body's metabolism. Your metabolism is how your body turns food into energy, which keeps the body's systems working properly.
The thyroid gland is part of the endocrine system, and the pituitary gland regulates it. The amount of thyroid hormones our body needs can vary. The pituitary gland sends signals to the thyroid to tell it to release more or fewer hormones into the bloodstream.
When your thyroid produces and releases more hormones than your body needs, it's known as hyperthyroidism. Hyperthyroidism speeds up your metabolism and can increase your heart rate, breathing, and digestion. It can also affect your mood, mental state, and weight.
The increase in metabolism caused by an overactive thyroid speeds up every operation and process of your body. As a result, people with hyperthyroidism often notice they seem to have more energy, accompanied by difficulty sleeping, increased sweating, hand tremors, and muscle weakness, especially in the upper arms and thighs. You may also notice changes in your hair texture or experience hair loss.
An overactive thyroid can speed up every function of the body and cause various symptoms, including:
A noticeable enlargement of the thyroid gland
Rapid or irregular heartbeat
Feelings of anxiousness, nervousness, or irritability
Frequent bowel movements or diarrhea
Menstrual and fertility issues
Thyroid disorders are common and can affect nearly anyone, including men, teenagers, children, and babies. Studies have found that over 12% of Americans¹ will develop a thyroid condition during their lifetime. While an underactive thyroid (hypothyroidism) is more common, hyperthyroidism affects nearly one in 100 Americans.²
It’s well-recognized by medical professionals that there is a complex relationship between thyroid disease, body weight, and metabolism.³ Basal metabolic rate or BMR measures metabolism by determining the amount of oxygen used over a specific period during rest.
Animal testing has found that a drug-induced (such as amphetamines) high BMR³ often changes energy balance, leading to weight loss. Scientists believe this same principle is why any changes in hormone levels lead to BMR and body weight changes. While many things influence BMR other than the thyroid, people with overactive thyroids tend to have high BMRs.
Additional research has discovered that thyroid medication can affect body weight.⁴ People with hypothyroidism and low hormone levels typically lose weight when they take medication to correct their condition. Conversely, people with hyperthyroidism tend to gain back lost weight with treatment.
While hyperthyroidism typically causes weight loss, it may cause weight gain in some circumstances. For example, weight gain may be caused by:
Along with increased metabolism and energy, hyperthyroidism usually increases appetite. So, if your body is taking in substantially more calories, you may gain weight even if your body is using more energy.
As hyperthyroidism is a thyroid disorder, your body will operate abnormally until you receive treatment to balance your hormones and return your body to a normal state. However, the thyroid will produce fewer hormones because of your treatment, so you might regain the weight that you lost before beginning treatment.
Anti-thyroid medications block your thyroid's ability to produce new hormones. These medications reduce the thyroid’s hormone production, causing a gradual reduction in symptoms over several weeks or months.
Beta-blockers block the action of thyroid hormones on the body, and doctors often combine them with another treatment option to control hyperthyroidism long term. Beta-blockers don't affect hormone levels but control symptoms such as rapid heartbeat, nervousness, and shakiness.
Radioactive iodine is an oral medication that your overactive thyroid cells absorb. The radioactive substance damages these cells, causing the thyroid gland to shrink, reducing thyroid hormone levels. These medications typically begin working in a few weeks and can destroy all or part of the thyroid gland, depending on the treatment you require. This effectively eliminates hyperthyroidism but usually means taking medication to maintain normal hormone levels.
Surgery eliminates hyperthyroidism by removing the entire thyroid gland. However, like radioactive iodine, this solution typically requires lifelong supplements to normalize hormone levels.
Untreated hyperthyroidism can lead to severe complications. If you notice swelling at the front of your neck, experience unusual sweating, unexplained weight loss, a rapid heartbeat, or any other signs associated with hyperthyroidism, it's essential to talk to your doctor. Since many hyperthyroidism symptoms can be associated with other conditions, it's vital to describe all of your symptoms in detail.
Weight gain from hyperthyroidism is uncommon but possible. It often starts after treatment begins as you regain the weight you lost from the condition. However, many other factors can cause weight gain. If you have hyperthyroidism and are gaining weight, speak to your doctor to determine the exact cause.
General information/press room | American Thyroid Association
Hyperthyroidism (Overactive Thyroid)
Thyroid and weight | American Thyroid Association
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