AbstractDNA fragmentation and apoptosis‐related proteins have been investigated in thyroid cells and there is evidence that Fas‐mediated apoptosis is inhibited by thyroid stimulating hormone (TSH). We investigated DNA fragmentation by terminal deoxynucleotidyl transferase‐mediated dUTP nick end labeling (TUNEL), and Bcl‐2 and Fas antigen expression by immunocytochemistry in skeletal muscles from 12 patients with hypothyroid myopathy and 5 patients with hyperthyroid myopathy. The finding of very few TUNEL‐positive muscle fibers in both conditions suggests that apoptosis does not play a role in the pathogenesis of thyroid myopathies. Bcl‐2 expression increased significantly in hypothyroid myopathy, correlating with high serum TSH levels, and not with either triiodothyronine (T3) or thyroxine (T4) serum levels. By contrast, Fas antigen was overexpressed in hyperthyroid myopathy, correlating with low TSH levels. These findings suggest an anti‐apoptotic role for TSH itself in skeletal muscle. © 2002 Wiley Periodicals, Inc. Muscle Nerve 26: 383–388, 2002