Weight Gain in Hypothyroidism
Weight gain in hypothyroidism is a modest increase in body weight (typically 5-8 kg) that occurs primarily due to decreased metabolic rate and accumulation of excess body water associated with myxedema, rather than true fat accumulation. 1, 2
Mechanism and Magnitude of Weight Gain
The weight gain is relatively modest: Men typically gain approximately 8.0 kg and women gain approximately 5.5 kg during the hypothyroid state. 3
The primary mechanism is fluid retention (myxedema), not fat accumulation—when hypothyroidism is treated with levothyroxine, weight loss occurs predominantly through excretion of excess body water, with fat mass remaining virtually unchanged. 4
Decreased resting energy expenditure (REE) contributes to the weight gain, as metabolic rate is reduced in the hypothyroid state. 4
Weight gain affects 24-59% of patients with hypothyroidism, making it one of the more variable symptoms. 5
Clinical Presentation
The weight gain in hypothyroidism presents alongside other metabolic symptoms:
Tendency toward weight gain with difficulty losing weight, accompanied by fatigue (68-83% of patients), cold intolerance, constipation, and mental slowing. 1, 2, 5
Physical examination findings include periorbital puffiness, coarse skin, and delayed ankle reflexes—all reflecting the myxedematous state. 1, 2
Critical Pitfall: Weight After Treatment
A common misconception is that levothyroxine treatment will reverse weight gain—this is often not the case. 6
Patients with inadequately controlled hypothyroidism (TSH >4.2 μIU/mL) continue to gain weight even on levothyroxine therapy: newly diagnosed patients gained an additional 3.02 kg over 2 years, and known hypothyroid patients gained 3.78 kg over 2 years despite treatment. 6
Only patients achieving adequate TSH control (≤4.2 μIU/mL) maintain stable weight—they neither lose nor gain significant weight during treatment. 6
Weight loss with levothyroxine treatment is primarily water loss, not fat loss, as fat mass remains unchanged despite normalization of thyroid function. 4
Patients treated for hyperthyroidism face increased obesity risk: there is a 1.7-fold increased risk in men and 1.3-fold increased risk in women of becoming obese (BMI >30 kg/m²) after treatment compared to the general population, representing an "overshoot" beyond premorbid weight. 3
Diagnostic Implications
Weight gain alone is non-specific and should prompt thyroid function testing (TSH and free T4) only when accompanied by other hypothyroid symptoms such as fatigue, cold intolerance, constipation, or depression. 1
Elevated TSH with low free T4 confirms overt hypothyroidism in patients presenting with unexplained weight gain and other metabolic symptoms. 1, 2, 5