Weakness

Muscle weakness is a lack of muscle strength. The causes are many and can be divided into conditions that have true or perceived muscle weakness. 

True weakness (or neuromuscular) describes a condition where the force exerted by the muscles is less than would be expected. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis.

Perceived weakness (or non-neuromuscular) describes a condition where a person feels more effort than normal is required to exert a given amount of force but actual muscle strength is normal, for example chronic fatigue syndrome.

In some conditions, such as myasthenia gravis, muscle strength is normal when resting, but true weakness occurs after the muscle has been subjected to exercise. This is also true for some cases of chronic fatigue syndrome, where objective post-exertion muscle weakness with delayed recovery time has been measured and is a feature of some of the published definitions.

Muscle weakness can also be classified as either "proximal" or "distal" based on the location of the muscles that it affects. Proximal muscle weakness affects muscles closest to the body's midline, while distal muscle weakness affects muscles further out on the limbs.

Proximal muscle weakness can be seen in Cushing's syndrome and hyperthyroidism.

Hyperthyroidism

Hyperthyroidism is the condition that occurs due to excessive production of thyroid hormone by the thyroid gland. Signs and symptoms vary between people and may include irritability, muscle weakness, sleeping problems, a fast heartbeat, heat intolerance, diarrhea, enlargement of the thyroid, and weight loss. Symptoms are typically less in the old and during pregnancy.