Another cause of myopathy is hypokalemia. Hypokalemia due to diuretic use or chronic inhalant exposure has been noted to cause this type of myopathy. Abuse of licorice, which contains glycyrrhizic acid, can also cause such a myopathy by its aldosterone-like effect and associated hypokalemia. Hyperthyroidism can cause proximal muscle weakness although the muscles most often involved in hyperthyroidism are those of the upper limbs. Associated clinical findings of hyperthyroidism are usually present.
Hypothyroidism also causes proximal muscle weakness.
This is usually associated with crampy pain of the muscles and other findings of hypothyroidism. Of interest, lithium does interfere with uptake by the thyroid of iodine. Specifically there is interference with the iodination of tyrosine and the synthesis of thyroxine. The patient's thyroxine level may be slightly decreased and the thyroid stimulating hormone increased. Most patients, however, remain euthyroid but may develop a goiter.