Hypothyroidism and Weight Gain: Mechanism, Magnitude, and Management

Hypothyroidism causes modest weight gain (typically 5–10 lbs), primarily from:

• Reduced basal metabolic rate (BMR): ↓ Thermogenesis → ↓ energy expenditure at rest.
• Fluid retention: Myxedematous infiltration (glycosaminoglycan accumulation) → non-pitting edema.
• Reduced lipolysis and increased lipogenesis.

Key Clarifications

• Weight gain is not solely from fat; much is fluid.
• Severe obesity is rarely caused by hypothyroidism alone.
• Treatment with levothyroxine normalizes BMR but does not automatically reverse weight — caloric balance still governs.

Management Approach

1. Optimize thyroid levels (TSH 0.5–2.5).
2. Assess diet, activity, sleep, stress, medications (e.g., beta-blockers, antidepressants).
3. Modest caloric deficit + resistance training preserves muscle during weight loss.
4. Screen for comorbid insulin resistance, PCOS, Cushing's if weight refractory.

Address thyroid first, then lifestyle. Realistic expectation: 10–15% of weight gain attributable to thyroid; rest requires behavioral intervention.