Coupled diminished energy turnover and phosphorylase a formation in contracting hypothyroid rat muscle

Effects of thyroid hormones on skeletal muscle bioenergetics. In vivo phosphorus-31 magnetic resonance spectroscopy study of humans and rats.

“…Furthermore, these investigators suggested that energy metabolism is more efficient in hypothyroidism. ATP turnover was found to be reduced in working human hypothyroid muscles (10, 1 1) and similar data were obtained from tetanized animal muscles (12,13). A reduction in mitochondrial activity and improved bioenergetics (by a factor of 3) is paradoxical and inconsistent with the fact that many hypothyroid patients complain of exercise intolerance and fatigue (1,2).…”

“…Earlier biochemical studies on muscle samples taken during rest and after maximal ischemic exercise showed that work-induced depletion of PCr and rise in Pi were less pronounced in hypothyroid muscle, both in human and animals (10)(11)(12)(13). These findings led to the suggestion that there is less ATP turnover in hypothyroid muscle compared with normal muscle during similar work loads, or that energy metabolism is more efficient in hypothyroidism.…”

“…This may have masked the contribution of defective oxidative metabolism to bioenergetic changes in hypothyroidism. Increased work loads were paradoxically not associated with a further decrease in PCr/Pi in these in vitro biochemical determinations (13). A marked rise in Pi was associated with minimal PCr depletion (12), which is not stoichiometric and suggests loss of PCr from the sample.…”

“…Muscle bioenergetic studies have been less conclusive in hyperthyroidism; ATP turnover was increased in a single case compared with controls (10), while rats showed no significant changes (12). Although hyperthyroidism may also present with exercise intolerance, it is not clear whether the biochemical findings relate to the clinical presentation.…”

Neuromuscular symptoms and dysfunction in subclinical hypothyroid patients: beneficial effect of L‐T4 replacement therapy

“…Serum levels of skeletal CPK are markedly increased in hypothyroidism in rough proportion to its severity (Burnett et al ., 1994); with treatment, CPK rapidly returns to normal, although muscle weakness is slower to disappear (Khaleeli & Edwards, 1984). Furthermore, glycogen accumulation and decreased activity of enzymes involved in energy production have been described in hypothyroid red (type I) fibres (Khaleeli et al ., 1983; Leijendekker et al ., 1985; Chu et al ., 1985). This is not surprising, since skeletal muscle is a target organ for thyroid hormones (Miller et al ., 1988).…”