- Fatigue is the most common clinical manifestation of hypothyroidism due to reduced basal metabolic rate and decreased energy production.
- Patients experience generalized tiredness and lethargy as a primary symptom.
- In contrast, weight loss and heat intolerance are typical of hyperthyroidism, while diarrhea is more associated with increased gut motility in thyrotoxicosis.
- Hypothyroidism commonly presents with cold intolerance, weight gain, and constipation, but fatigue remains the hallmark symptom due to systemic slowing of metabolism.

Reference: Davidson’s Principles and Practice of Medicine, 24th Edition, Chapter: Disorders of the Thyroid Gland, Page 539.