What is hyperthyroidism?

Hyperthyroidism¹ occurs when the thyroid gland makes more thyroid hormone than the body needs. If you have hyperthyroidism, you might feel anxious, or tired or have shaky hands, a rapid heartbeat, and unexpected weight loss.

Hyperthyroidism refers to an overactive thyroid gland. The thyroid is a small, butterfly-shaped gland found at the base of the neck. It produces hormones that control the way your body uses energy (a process called metabolism). The two most important thyroid hormones produced by the thyroid gland are thyroxine (T4) and triiodothyronine (T3).

Hyperthyroidism² can accelerate the metabolism, causing symptoms including unintentional weight loss and a rapid or irregular heartbeat (arrhythmia). It is often associated with an autoimmune condition known as Graves’ disease and inflammation of the thyroid (thyroiditis).

Several treatments are available for hyperthyroidism. Medications are used to block and slow the production of thyroid hormone, and surgery to remove all or part of the thyroid may be used in severe cases.

While untreated hyperthyroidism can be a serious health concern, most people with the condition respond well to treatments.

How common is hyperthyroidism?

About 1.2% of people³ in the United States have hyperthyroidism. Women are 2 to 10 times more likely than men to develop hyperthyroidism.

You are also more likely to get hyperthyroidism if you have a family history of thyroid disease, consume large amounts of iodine, are older than age 60, or if you were pregnant within the past 6 months.

Hypothyroidism vs. Hyperthyroidism

You may have also heard of a condition called hypothyroidism. This condition, which causes an underactive thyroid, occurs when the gland does not make enough thyroid hormone. In other words, hypothyroidism can be thought of as the opposite of hyperthyroidism.

To help you remember the difference between these two conditions, remember that hyper- means⁴ “excess or overabundance,” while hypo- means “less than normal or deficient.”

Hashimoto’s thyroiditis is the most common cause⁵ of hypothyroidism. This condition is an autoimmune disorder, meaning it causes the body to attack its own immune system. This results in the thyroid unable to produce hormones as it should.

Hyperthyroidism causes

Hyperthyroidism may be caused by a number of health conditions, including Graves' disease, thyroiditis (particularly, Hashimoto's thyroiditis), and Plummer's disease.

Consuming too much iodine (a substance found in certain foods, such as seaweed and iodized salt) or taking too much thyroid hormone medication may also raise your thyroid hormone levels.

The thyroid produces two main hormones: thyroxine (T4) and triiodothyronine (T3). These hormones regulate many different processes and factors in the body, such as metabolic rate, body temperature, heart rate, and protein production.

The thyroid also produces calcitonin, a hormone that helps regulate the amount of calcium in your blood.

Normally, a healthy thyroid releases the right amount of hormones to keep the body well-regulated. In people with hyperthyroidism, however, the thyroid produces too much T4. This overproduction of T4 may occur for a number of reasons, including:

• Graves' disease: Graves' disease is an autoimmune disorder that causes an excess of T4 to be produced. This disease is the most common cause of hyperthyroidism.
• Thyroiditis: Thyroiditis is a condition in which the thyroid becomes inflamed. This inflammation can cause excess hormones stored in the thyroid to leak into the bloodstream. Thyroiditis can develop as a result of a number of factors, including an autoimmune condition, pregnancy, or, in some cases, unknown reasons.
• Hyperfunctioning thyroid nodules: This condition, also referred to as toxic adenoma, toxic multinodular goiter, or Plummer's disease, is a form of hyperthyroidism that occurs when benign (non-cancerous) lumps in the thyroid called adenomas produce too much T4.

Hyperthyroidism risk factors

Some health conditions and factors may increase your likelihood for developing hyperthyroidism. Risk factors⁶ for this condition include:

• A family history of thyroid conditions—particularly Graves' disease
• Female sex
• Chronic illnesses, such as type 1 diabetes, pernicious anemia, and primary adrenal insufficiency

What is Graves’ disease?

Graves’ disease, a common cause of hyperthyroidism, is an immune system disorder that causes the thyroid to produce excess thyroid hormones. About one in 200 people⁷ in the United States has Graves’ disease.

Graves' disease results from⁸ a malfunction in the body's disease-fighting immune system. However, the exact reason for this malfunction is still unknown.

The common signs and symptoms⁹ of Graves’ disease often overlap with those of hyperthyroidism. They can include:

• Anxiety and irritability
• A fine tremor (shaking) in the hands or fingers
• Weight loss, despite normal eating habits
• Enlargement of the thyroid gland (called a goiter)
• Fatigue, tiredness, or muscle weakness
• Red, thickened skin on the shins or tops of the feet (Graves’ dermopathy or pretibial myxedema)—usually painless and mild, but can be painful in some people
• Rapid or irregular heartbeat (palpitations)
• Trouble sleeping

Roughly 30% percent of people with Graves' disease also present at least some signs and symptoms of a condition called Graves' ophthalmopathy¹⁰. Graves' ophthalmopathy (also called thyroid eye disease) affects the muscles and other tissues around your eyes, causing dryness and irritation.

The common signs and symptoms of Graves' ophthalmopathy may affect one or both eyes and include:

• Dry eyes
• Red or swollen eyes
• Excessive tearing
• Discomfort and inflammation
• Light sensitivity
• Blurred or double vision (diplopia)
• Reduced eye movement
• Protruding eyeballs

The main symptoms of Graves’ ophthalmopathy often go away¹¹ over time. Some of the signs and symptoms¹² of thyroid eye disease may come on quickly, while others may develop slowly. They also may appear years before or after the onset of hyperthyroidism.

Hyperthyroidism symptoms

The signs and symptoms of hyperthyroidism can overlap with those of other health conditions. This can make it difficult for your provider to properly diagnose the condition.

Hyperthyroidism can cause a wide variety of signs and symptoms¹³, including:

• Unintentional weight loss, even if your appetite and food intake stay the same or increase
• Rapid heartbeat (tachycardia)—commonly defined as a heartbeat faster than 100 beats per minute
• Irregular heartbeat (arrhythmia)
• Pounding of your heart (palpitations)
• Increased appetite
• Nervousness, anxiety, and irritability
• Tremor—a fine trembling in the hands and fingers
• Excessive sweating
• Changes in menstrual cycle
• Increased heat sensitivity
• Changes in bowel patterns, especially more frequent bowel movements
• An enlarged thyroid (goiter), which may appear as a swelling at the base of your neck
• Fatigue and muscle weakness
• Difficulty sleeping
• Thin or weakened skin
• Fine, brittle hair

Older adults with hyperthyroidism are more likely to experience very mild or no signs or symptoms of the condition. These can include increased heart rate, sensitivity to heat (heat intolerance), and the tendency to become tired during ordinary activities.

Complications from hyperthyroidism

Hyperthyroidism can lead to a number of health complications¹⁴, especially when left untreated. These may include:

• Brittle bones: Untreated hyperthyroidism can lead to a condition called osteoporosis, which causes weak, brittle bones. A large part of the strength of your bones depends upon the amount of calcium and other minerals they contain. Excess thyroid hormone reduces the body's ability to incorporate calcium into your bones, causing them to weaken.
• Eye problems: Some people with Graves' disease, an autoimmune disease that commonly causes hyperthyroidism, develop Graves’ ophthalmology. This condition causes various eye problems, including dryness, bulging, redness and swelling, sensitivity to light, and blurred or double vision (diplopia). If left untreated, severe eye problems can lead to vision loss.
• Red, swollen skin: A small portion of people with Graves' disease develop a condition known as Graves' dermopathy. This condition causes redness and swelling on the skin, often on the shins and feet.
• Heart problems: Some of the most critical complications of hyperthyroidism are those that affect the heart. Hyperthyroidism may lead to a rapid heart rate, a heart rhythm disorder that increases your risk of stroke (atrial fibrillation), and congestive heart failure, a condition in which your heart can't circulate enough blood to meet your body's needs.
• Thyrotoxic crisis: Having hyperthyroidism puts you at risk for a sudden intensification of your symptoms called thyrotoxic crisis. This can lead to fever, a rapid pulse, and even delirium. If this occurs, seek immediate medical care.

Hyperthyroidism in cats

Hyperthyroidism is, perhaps surprisingly, a common disease in cats¹⁵. In fact, hyperthyroidism affects 10% of cats¹⁶ over 10 years of age.

Common clinical signs¹⁷ of feline hyperthyroidism include weight loss, increased appetite, and increased thirst and urination. Other signs and symptoms include vomiting, diarrhea, and hyperactivity. Cats with hyperthyroidism may also have coats that appear unkempt, matted, or greasy.

If your veterinarian suspects that your cat may have a thyroid problem, they will conduct a physical examination that includes feeling the cat’s neck area to check for an enlarged thyroid (goiter). He or she may also check the cat’s heart rate and blood pressure, which can indicate changes in metabolic activity.

If thyroid disease is a possibility for your cat, your veterinarian will likely conduct blood tests and analyze your cat’s thyroid hormone levels.

Feline hyperthyroidism can be treated through medication, radioactive iodine therapy, surgery, and dietary therapy.