Thyrotoxic myopathy

Symptoms and signs

Physical symptoms include muscle weakness, degeneration of muscle tissue, fatigue, and heat intolerance. Chronic TM develops slowly, with patients reporting decreased exercise tolerance and increased fatigue after about six months. Untreated chronic TM may lead to difficulty swallowing and respiratory distress, though these are rare.

Acute TM, rarer than chronic, appears within days and causes rapid muscle fiber degradation, severe cramps, and pain. Some acute patients experience blurred vision and bulging eyes from eye muscle degradation. Acute TM often results in severe respiratory failure.

Pathophysiology

Excess thyroxine is believed to trigger thyrotoxic myopathy and cause muscle tissue degradation. Thyroxine, produced by the thyroid gland, regulates nervous system growth metabolism and basal metabolic rate. Scientists agree that thyroxine degrades muscle fibers at the motor end plates of neuromuscular junctions, though debate exists whether degradation occurs from the muscular side, the nervous side, or both. Normally, muscle contraction involves acetylcholine release from the axon terminal, which stimulates the motor end plate, leading to calcium release and contraction.

In TM, structural changes in motor end plates may cause fiber degradation. Research shows decreased levels of acetylcholinesterase within the neuromuscular junction, blocking acetylcholine breakdown and causing overstimulation of the motor end plate. This overstimulation may lead to muscle fatigue, weakness, and degradation. The decrease in acetylcholinesterase and structural changes may result from thyroxine blocking axoplasmic flow of trophic factors down the axon terminal.

Diagnosis

A neurologist experienced in neuromuscular disorders typically diagnoses thyrotoxic myopathy. Electromyography compares muscle contraction responses to electrical stimulus; results may show normal or myopathic responses depending on disease progression.

Blood tests reveal increased thyroxine levels, which together with decreased neuromuscular responses provide strong evidence for TM. Creatine phosphokinase levels are examined; normal levels may indicate early stages, while increased levels suggest later progression. Muscle biopsies may be taken to assess physical degradation, showing normal to severe fiber degradation corresponding to progression severity.

Treatment

Treatment involves collaboration among a neuromuscular specialist, endocrinologist, surgeon, and ophthalmologist; a physical therapist may be consulted for rehabilitation if significant muscle degradation occurs. The goal is to reduce thyroxine overproduction and restore thyroid homeostasis through medication, radiation, or surgery. Medications include beta-blockers to alleviate symptoms (without reducing damage) and propylthiouracil or methimazole to block thyroxine release and its damage to muscle tissue.

Radioactive iodine destroys the overactive thyroid gland, as the gland cannot distinguish it from normal iodine; one dose typically blocks thyroxine overproduction, but the gland is completely destroyed, often leading to hypothyroidism within months or up to 20–30 years, requiring lifelong thyroid replacement hormones. Surgical removal of portions of the thyroid is another option, often performed when overproduction is caused by a toxic multinodular goiter, addressing both aesthetic and physiological effects.

Did You Know?

• The most common causes of hyperthyroidism are Graves’ disease, toxic multinodular goiter, and autonomously functioning thyroid adenoma.
• While hyperthyroidism is more common in women, the development of thyrotoxic myopathy is more common among men with hyperthyroidism.
• Case studies show that only about half of patients with diagnosed hyperthyroidism complained of symptoms characteristic of TM.