Thyrotoxic Myopathy

What is Thyrotoxic Myopathy?

Thyrotoxic myopathy is a rare neuromuscular disorder that occurs as a result of excessive production of thyroxine, a hormone produced by the thyroid gland. It is commonly associated with hyperthyroidism, also known as an overactive thyroid gland.

Who is affected?

Although women are more commonly affected by hyperthyroidism than men, it is more common for men to develop thyrotoxic myopathy.

What causes thyrotoxic myopathy?

There are two main causes of thyortoxic myopathy. These are:

• Multinodular goitre: this occurs when the thyroid gland becomes enlarged due to the growth of nodules. This results in increased production of thyroxine
• Graves’ disease: this is an autoimmune condition that occurs when the immune system attacks the thyroid gland, causing it to produce more thyroxine than normal. Autoimmune disorders occur when the body’s immune system mistakes healthy cells for a harmful threat and starts to attack them.

What are the symptoms?

Symptoms of thyrotoxic myopathy include:

• weakness in the muscles
• tenderness in the muscles
• loss of muscle strength and mass in the shoulder muscles and pelvis
• decreased tolerance to heat
• fatigue
• rhabdomyolysis (sudden, rapid breakdown of muscle tissue)
• bouts of severe weakness in the muscle possibly caused by decreased potassium levels
• blurred vision (this is less common and may occur if the muscles that control the eyes are affected)

Treating thyrotoxic myopathy

Treatment is focused on restoring levels of thyroxine hormone to normal. This may be achieved by medication, iodine radiation and surgery to remove some or all of the thyroid gland. Your doctor will discuss the relevant options with you when they have your test results.

What is the outlook?

Treatment can be very effective in reducing muscle weakness. However, it is important that treatment is administered early, as complications can be very serious.