Hypothyroidism combined with anti-signal recognition particle immune-mediated necrotizing myopathy: A case report and review of the literature.

Immune-mediated necrotizing myopathy (IMNM) is characterized by proximal limb weakness, elevated creatine kinase (CK) levels, and myofiber necrosis without or with only a small amount of inflammatory cell infiltrate. There is only 1 report of hypothyroidism combined with antibody-negative IMNM to date. We aimed to describe a rare case of hypothyroidism combined with anti-signal recognition particle (SRP) IMNM for the first time and review the previous literature. A 50-year-old male, who had a 4-year history of hypothyroidism treated with levothyroxine replacement therapy, presented with progressive symmetrical proximal muscle weakness. Laboratory testing showed an elevated CK level of 6,106 U/L. Electrophysiological examination elicited carpal tunnel syndrome and myogenic damage. Muscle MRI revealed diffuse abnormal signals in both lower limbs. Given that muscle symptoms are widely recognized among hypothyroid patients, hypothyroid myopathy was initially suspected, and thyroid hormone tablets were added for a week. However, muscle weakness persisted along with an even higher CK (7,020 U/L). Quadriceps muscle biopsy was performed and indicated inflammatory myopathy. Myositis specific antibodies (MSAs) detection revealed that anti-SRP was positive. A diagnosis of hypothyroidism combined with anti-SRP IMNM was finally made. Treatment of corticosteroid and immunosuppressive agents achieved a positive clinical and biochemical response. This case indicates that hypothyroidism combined with anti-SRP IMNM is a rare clinical entity, possibly caused by a general immunologic dysregulation.