Thyroid dysfunction:
- Hypothyroid myopathy: Characterized by slowed muscle contraction and relaxation. This may be caused by a shift in the distribution of muscle fiber types from fast-twitch fibers to slow-twitch fibers and diminished energetic consumption, causing muscle weakness, fatigue, and exertional pain.
- Thyrotoxic myopathy: due to over stimulation of muscle fibers causing fatigue, weakness, and finally degradation, as a result of increased mitochondrial respiration and enhanced beta-adrenergic sensitivity. could also be due to accelerated protein degradation and lipid oxidation.
Adrenal dysfunction:
- Adrenal insufficiency: Contributing factors to muscle weakness include circulatory insufficiency, fluid and electrolyte imbalance, impaired carbohydrate metabolism, and starvation. However, myopathy is not likely to be a presenting finding.
Parathyroid dysfunction:
- Hyperparathyroidism: muscle weakness is due to reduced neuronal excitability. High plasma calcium decreases the permeability of neuronal membranes to sodium ions, decreases the ease with which action potentials can be initiated.
(in contrast with hypoparathyroidism, in which low plasma calcium increases neuronal excitability hence tapping nerves, not muscles, will further exaggerates neuronal excitability, eliciting Chvostek's and Trousseau's signs)
