Hypothyroid myopathy-pathological and ultrastructural study.

Seventeen patients with hypothyroid myopathy were studied before and after thyroxine (T4) treatment. The severity of clinical myopathy was assessed with the aid of a modified rating scale. Laboratory findings including thyroid function, serum creatine kinase (CK), and electromyography were assessed at regular time intervals until a final muscle biopsy was performed. The average period of follow-up was 1.8 years. The authors emphasize: 1) in skeletal muscle pathology of hypothyroidism, the fiber atrophy and increased central nuclear counts are evidence of clinical myopathy during thyroxine treatment; 2) in ultrastructural pathology, the abnormal glycogen accumulation accounts largely for clinical severity and its ongoing resolution is parallel to steady amelioration following T4 therapy, while mitochondrial abnormalities are insignificant in clinical correlation and probably become permanent in some cases with prolonged hypothyroidism; and 3) serial needle biopsies of skeletal muscle are impractical for long-term study of hypothyroid myopathy, but they may be reserved for those patients with a sustained myopathic complaints on T4 therapy.