MYOPATHY AS THE PERSISTENTLY ISOLATED SYMPTOMATOLOGY OF PRIMARY AUTOIMMUNE HYPOTHYROIDISM

Although disorders of thyroid function may cause a wide range of muscl e disturbances, an overt myopathy has been rarely reported as an isola ted clinical presentation of hypothyroidism. We observed 10 patients ( 5 males and 5 females) who had been referred to the department of neur ology because of muscular fatigability, myalgia, cramps, or proximal w eakness. Laboratory investigation showed that all patients had hypothy roidism due to Hashimoto's thyroiditis (atrophic variant in 9/10). Cla ssic symptoms/signs of hypothyroidism such as lethargy, constipation, cold intolerance, myxedematous facies, and/or bradycardia were absent, as assessed independently by the three coauthoring thyroidologists. M uscular complaints improved greatly and then disappeared after substit utive levothyroxine treatment. Muscle biopsy revealed nonspecific chan ges. Nicotinamide adenine dinucleotide reductase (NADH-TR)-hyporeactiv e cores were present in two patients (10% and 90% of type 1 fibers). O n electron microscopy, the core areas showed disorganized myofibrils, Z-band streaming, rod formation, and paucity of mitochondria and glyco gen granules. Desmin intermediate filaments were overexpressed only in some cores. The similarity of the pattern of desmin expression betwee n hypothyroid cores and target lesions of denervated fibers supports t he hypothesis that, at least in some of our patients, myopathy was the result of an impaired nerve-mediated action of thyroid hormones on sk eletal muscle. Our observations suggest that an isolated myopathy as t he sole manifestation of hypothyroidism is not a rare event. We postul ate that our cases may constitute a peculiar subgroup of Hashimoto's t hyroiditis patients: (1) the strikingly abnormal F/M ratio of 1:1; (2) the relatively younger age; (3) the rarity of the goitrous variant; ( 4) the unusual finding of antithyroglobulin (Tg-Ab)> antithyroid perox idase (TPO-Ab). Thorough evaluation of thyroid function is appropriate in patients with myopathy of uncertain origin.