Polymyositis-like hypothyroid myopathy: diagnostic challenges and therapeutic outcomes in a case series.

All 10 patients with polymyositis-like hypothyroid myopathy presented with proximal muscle weakness, myalgia, and/or arthralgia accompanied by elevated muscle enzymes.

• Case series included 10 patients, 70% female, with mean age 46.1 ± 10.3 years.
• All patients had elevated CK, CK-MB, and LDH.
• All patients tested negative for myositis antibodies.
• All patients exhibited thyroid dysfunction (decreased fT3/fT4, elevated TSH) and positive thyroglobulin antibodies (100%).

Polymyositis-like hypothyroid myopathy was associated with multisystem manifestations including liver dysfunction, hyperlipidemia, and serous effusions.

• Liver dysfunction was present in 70% of patients.
• Hyperlipidemia was present in 70% of patients.
• Pericardial effusion was present in 70% of patients.
• These multisystem features contributed to diagnostic challenges by mimicking polymyositis.

Muscle biopsies in these patients revealed degeneration and atrophy without necrosis or inflammation.

• The absence of necrosis or inflammation on biopsy distinguished hypothyroid myopathy from true polymyositis.
• This histological finding was consistent across the case series.
• Biopsy findings supported a metabolic rather than inflammatory etiology.

After 3 months of thyroxine replacement therapy, symptoms significantly improved and multiple laboratory parameters decreased significantly.

• Thyroxine replacement therapy was administered at 100–150 µg/day for 3 months.
• Symptom improvement was statistically significant (P < 0.05).
• Reductions in ALT, AST, CHOL, CK, CK-MB, and LDH were all statistically significant (P < 0.01).
• fT3/fT4 levels normalized following treatment.

Muscle enzyme levels correlated negatively with fT3/fT4 and positively with TSH.

• Negative correlation between muscle enzymes and fT3/fT4: rs = -0.76 to -0.78, P < 0.05.
• Positive correlation between muscle enzymes and TSH: rs = 0.70 to 0.72, P < 0.05.
• These correlations suggest that the degree of thyroid dysfunction is directly associated with the severity of muscle enzyme elevation.

Hypothyroid myopathy affects 30–80% of hypothyroid patients and frequently mimics polymyositis, posing significant diagnostic challenges.

• The overlapping clinical features include proximal muscle weakness and elevated muscle enzymes seen in both conditions.
• The absence of myositis-specific antibodies is a key distinguishing feature of hypothyroid myopathy.
• Thyroid screening was identified as important even in the absence of classic hypothyroidism symptoms.