Is low thyroid hormone signaling linked to fatigue, weight gain, mood changes, and muscle weakness or aches?

Clinical and metabolic evidence

Low thyroid hormone signaling is strongly linked to clinical fatigue and weight gain, which are primary features of both overt and subclinical hypothyroidism (SCH). Thyroid hormones act as central coordinators of the body's basal metabolic rate (BMR). When thyroid hormone levels or tissue receptivity fall, mitochondrial oxidative metabolism and cellular energy production decrease, resulting in profound fatigue. Even in mild or subclinical cases (SCH), patients frequently report significant fatigue. Similarly, a diminished BMR leads to reduced energy expenditure and altered lipid metabolism, which commonly manifests as weight gain. Although the weight gain associated with subclinical thyroid decline is typically modest, it is highly prevalent, particularly in middle-aged women.

Neuropsychiatric and musculoskeletal manifestations

• Mood changes: Low thyroid hormone signaling is closely linked to mood disturbances, particularly depressive symptoms. Large-scale meta-analyses demonstrate that individuals with subclinical hypothyroidism have a significantly higher risk of developing depression compared to euthyroid controls. This association is especially pronounced in individuals around the age of 50.
• Muscle weakness and aches: Muscle weakness, stiffness, and myalgia (often termed hypothyroid myopathy) affect up to 80% of individuals with low thyroid signaling. These symptoms impair physical capacity but are highly responsive to thyroid hormone replacement therapy (levothyroxine).

Mechanistic explanations

At the cellular level, thyroid hormones are critical regulators of mitochondrial biogenesis and function:

• Mitochondrial dysfunction: Thyroid hormones bind to nuclear thyroid hormone receptors ($\text{TR}\alpha$ and $\text{TR}\beta$), which drive the transcription of genes responsible for mitochondrial replication and oxidative capacity.
• AMPK-SIRT1-PGC-1$\alpha$ pathway disruption: Low thyroid hormone signaling downregulates key metabolic pathways, such as the AMPK-SIRT1-PGC-1$\alpha$ signaling axis, which is vital for maintaining cellular energy balance.
• ATP depletion and cellular stress: In skeletal muscle, this pathway disruption leads to mitochondrial dysfunction, characterized by impaired ATP production and increased reactive oxygen species (ROS). The resulting energy deficit in muscle fibers directly drives muscle aches, stiffness, and weakness.

Bottom line

Low thyroid hormone signaling is clinically and mechanistically associated with fatigue, weight gain, depressive mood changes, and muscle weakness or aches. These symptoms are driven by systemic metabolic downregulation and skeletal muscle mitochondrial dysfunction, both of which are highly responsive to appropriate thyroid hormone replacement.