Two patients with proximal muscle weakness and marked elevations of serum muscle enzymes were initially believed to have polymyositis; however electromyography and muscle biopsies were normal. Both patients were subsequently found to have hypothyroidism. Each regained her muscle strength, and serum enzymes normalized with thyroid hormone replacement. Because muscle weakness and an elevated creatine phosphokinase occur in most patients with hypothyroidism, thyroid dysfunction must be considered in the differential diagnosis of polymyositis.Polymyositis is characterized clinically by symmetric proximal muscle weakness and, less frequently, by muscle pain and tenderness. Chemically, the levels of Submitted for publication February 10, 1976: accepted April 19, 1976 serum enzymes concentrated in skeletal muscle are almost invariably elevated. It is not well appreciated that hypothyroidism can present as a muscular syndrome indistinguishable both clinically and chemically from polymyositis, with few of the stigmata usually associated with thyroid hormone deficiency.It is the purpose of this report to describe 2 patients initially believed to have polymyositis on the basis of severe proximal myopathy with profound serum muscle enzyme elevations. Both were subsequently found to have primary hypothyroidism. The discriminating clinical and laboratory features are discussed, and the muscular abnormalities associated with hypothyroidism are reviewed. CASE REPORTS Patient 1RL, a 34-year-old Latin American housewife, was admitted to The Wilford Hall USAF Medical Center for evaluation of progressive proximal limb-girdle muscle pain and weakness of 4 months duration. Past health had been good, except for depressive symptoms of 3 years duration. In addition to muscular pain and weakness, she complained of paresthesias of the first four fingers of both hands and of periorbital swelling not associated with skin rash. There was no history of articular pain or of swelling, alopecia, or sicca symptoms.Physical examination disclosed an obese, chronically ill appearing white female. The pulse was 68/minute, blood pressure 110/60 mm Hg. and temperature normal. The skin revealed periungual erythema, and the hair was fine and dry. The thyroid gland was neither enlarged nor nodular. All