Hyperthyroidism can mimic other health problems, which can make it difficult for your doctor to diagnose. It can also cause a wide variety of signs and symptoms, including:

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

Older adults are more likely to have either no signs or symptoms or subtle ones, such as an increased heart rate, heat intolerance and a tendency to become tired during ordinary activities.

Graves' ophthalmopathy

Sometimes an uncommon problem called Graves' ophthalmopathy may affect your eyes, especially if you smoke. This disorder makes your eyeballs protrude beyond their normal protective orbits when the tissues and muscles behind your eyes swell. Eye problems often improve without treatment.

Signs and symptoms of Graves' ophthalmopathy include:

• Dry eyes
• Red or swollen eyes
• Excessive tearing or discomfort in one or both eyes
• Light sensitivity, blurry or double vision, inflammation, or reduced eye movement
• Protruding eyeballs

Thyroid gland

Hyperthyroidism can be caused by a number of conditions, including Graves' disease, Plummer's disease and thyroiditis.

Your thyroid is a small, butterfly-shaped gland at the base of your neck, just below your Adam's apple. The thyroid gland has an enormous impact on your health. Every aspect of your metabolism is regulated by thyroid hormones.

Your thyroid gland produces two main hormones, thyroxine (T4) and triiodothyronine (T3), that influence every cell in your body. They maintain the rate at which your body uses fats and carbohydrates, help control your body temperature, influence your heart rate, and help regulate the production of protein. Your thyroid also produces a hormone that helps regulate the amount of calcium in your blood (calcitonin).

Reasons for too much thyroxine (T4)

Normally, your thyroid releases the right amount of hormones, but sometimes it produces too much T4. This may occur for a number of reasons, including:

• Graves' disease. Graves' disease is an autoimmune disorder in which antibodies produced by your immune system stimulate your thyroid to produce too much T4. It's the most common cause of hyperthyroidism.
• Hyperfunctioning thyroid nodules (toxic adenoma, toxic multinodular goiter or Plummer's disease). This form of hyperthyroidism occurs when one or more adenomas of your thyroid produce too much T4. An adenoma is a part of the gland that has walled itself off from the rest of the gland, forming noncancerous (benign) lumps that may cause an enlargement of the thyroid.
• Thyroiditis. Sometimes your thyroid gland can become inflamed after pregnancy, due to an autoimmune condition or for unknown reasons. The inflammation can cause excess thyroid hormone stored in the gland to leak into your bloodstream. Some types of thyroiditis may cause pain, while others are painless.

Risk factors for hyperthyroidism, include:

• A family history, particularly of Graves' disease
• Female sex
• A personal history of certain chronic illnesses, such as type 1 diabetes, pernicious anemia and primary adrenal insufficiency

Hyperthyroidism can lead to a number of complications:

• Heart problems. Some of the most serious complications of hyperthyroidism involve the heart. These include a rapid heart rate, a heart rhythm disorder called atrial fibrillation that increases your risk of stroke, and congestive heart failure — a condition in which your heart can't circulate enough blood to meet your body's needs.
• Brittle bones. Untreated hyperthyroidism can also lead to weak, brittle bones (osteoporosis). The strength of your bones depends, in part, on the amount of calcium and other minerals they contain. Too much thyroid hormone interferes with your body's ability to incorporate calcium into your bones.
• Eye problems. People with Graves' ophthalmopathy develop eye problems, including bulging, red or swollen eyes, sensitivity to light, and blurring or double vision. Untreated, severe eye problems can lead to vision loss.
• Red, swollen skin. In rare cases, people with Graves' disease develop Graves' dermopathy. This affects the skin, causing redness and swelling, often on the shins and feet.
• Thyrotoxic crisis. Hyperthyroidism also places you at risk of thyrotoxic crisis — a sudden intensification of your symptoms, leading to a fever, a rapid pulse and even delirium. If this occurs, seek immediate medical care.