Can a patient with hyperthyroidism (an overactive thyroid gland) experience weight gain?

Hyperthyroidism and Weight Gain: Understanding the Relationship

Yes, patients with hyperthyroidism can experience weight gain, particularly during and after treatment, with more than half of treated patients experiencing excessive weight gain of ≥10% above their pre-disease weight. 1

Clinical Presentation of Hyperthyroidism

Hyperthyroidism typically presents with:

• Weight loss (in approximately 90% of untreated patients)
• Increased appetite
• Heat intolerance
• Tremors
• Anxiety
• Tachycardia
• Warm, moist skin
• Fatigue and muscle weakness 2

However, the clinical presentation can vary significantly between patients.

Weight Changes in Hyperthyroidism

During Active Hyperthyroidism

• While weight loss is the classic symptom of untreated hyperthyroidism, approximately 10% of patients may actually gain weight even during active disease 3
• This variability in weight response demonstrates that hyperthyroidism affects metabolism differently among individuals

During and After Treatment

• Weight gain is a common and expected outcome of successful hyperthyroidism treatment
• Research shows that men gain an average of 8.0 kg and women gain 5.5 kg during treatment 1
• Significant finding: This weight gain often exceeds the original weight loss, resulting in what patients describe as weight "overshoot" 1, 3

Mechanisms of Weight Gain in Treated Hyperthyroidism

Several factors contribute to weight gain during and after hyperthyroidism treatment:

1. Normalization of Metabolic Rate:

   • Treatment reduces the elevated metabolic rate caused by excess thyroid hormones 4, 5
   • Resting energy expenditure (REE) decreases significantly (from 1,630 to 1,484 kcal/24h in one study) 5

2. Food Energy Overconsumption:

   • Patients often continue their increased food intake pattern established during active hyperthyroidism 4
   • This creates a caloric surplus as metabolism normalizes

3. Treatment-Induced Hypothyroidism:

   • Radioactive iodine (RAI) treatment is associated with additional weight gain (0.6 kg more) compared to antithyroid drugs alone 1
   • Patients who develop hypothyroidism after treatment gain even more weight (predicted excess of 1.8 kg) 1
   • Episodes of elevated TSH (>10 mIU/L) or low free T4 (≤10 pmol/L) during follow-up are associated with additional weight gain 1

4. Changes in Body Composition:

   • Treatment leads to increases in both lean body mass and fat mass, though the timing and proportion of these changes varies 3

Risk Factors for Excessive Weight Gain

Certain factors predict greater weight gain after hyperthyroidism treatment:

• Severity of initial hyperthyroidism (higher free T4 at diagnosis) 6
• Greater disease-related weight loss before treatment 6
• Treatment with radioactive iodine versus antithyroid drugs alone 1
• Development of hypothyroidism during or after treatment 1
• Underlying Graves' disease 3

Clinical Implications

1. Obesity Risk:

   • After treatment, patients with hyperthyroidism have a significantly increased risk of becoming obese compared to the general population
   • Men have 1.7 times higher risk and women have 1.3 times higher risk 1

2. Patient Counseling:

   • Patients should be informed about the likelihood of weight gain before starting treatment
   • The potential for weight gain exceeding pre-disease weight should be discussed 3

3. Monitoring and Management:

   • Close monitoring of thyroid function to avoid periods of hypothyroidism
   • Consider dietary interventions, which have shown promise in limiting excessive weight gain 3
   • Regular weight monitoring throughout treatment and follow-up

Conclusion

While weight loss is the classic symptom of untreated hyperthyroidism, weight gain is a common and often significant consequence of treatment. This weight gain frequently exceeds the original weight loss, resulting in a higher weight than before the onset of hyperthyroidism. Healthcare providers should counsel patients about this expected outcome and consider strategies to mitigate excessive weight gain, particularly in those with risk factors for greater weight increases.