Can Synthroid (levothyroxine) cause weight gain?

Can Synthroid (Levothyroxine) Cause Weight Gain?

Contrary to common belief, Synthroid (levothyroxine) treatment can be associated with weight gain in some patients, particularly when thyroid function is not optimally controlled. 1, 2

Weight Changes with Levothyroxine Treatment

• Patients with hypothyroidism often experience weight gain as a symptom of their condition (24-59% of patients), and expect weight loss when starting treatment 3
• However, research shows that adequate treatment with levothyroxine does not necessarily lead to weight reduction; patients may either maintain their weight or continue to gain weight 1
• In a retrospective study, patients with uncontrolled hypothyroidism (TSH >4.2 μIU/mL) continued to gain weight over time despite levothyroxine treatment, with mean increases of 1.4-3.0 kg over one year and up to 3.8 kg over two years 1
• Even patients with well-controlled thyroid function (TSH ≤4.2 μIU/mL) did not experience significant weight loss with levothyroxine treatment 1

Mechanisms of Weight Gain with Levothyroxine

• Weight gain can occur due to several mechanisms:
  • Overtreatment: About 25% of patients on levothyroxine are unintentionally maintained on doses high enough to suppress TSH completely, which can lead to iatrogenic hyperthyroidism 4, 5
  • Undertreatment: Inadequate dosing may not fully address the metabolic effects of hypothyroidism 5
  • Fluctuating thyroid levels: Both markedly increased TSH (>10 mIU/L) and reduced free T4 (≤10 pmol/L) during treatment are associated with weight gain 2

Risk Factors for Weight Gain

• Treatment with radioactive iodine (131I) followed by levothyroxine replacement is associated with more weight gain (additional 0.6 kg) compared to treatment with antithyroid drugs alone 2
• Initiation of levothyroxine after radioactive iodine treatment is associated with further weight gain (0.4 kg), with a predicted excess weight gain of 1.8 kg in radioiodine-induced hypothyroidism 2
• Patients treated for hyperthyroidism have a significantly increased risk of becoming obese compared to the general population (odds ratio = 1.7 for men, 1.3 for women) 2

Monitoring and Management

• Regular monitoring of TSH levels every 6-8 weeks while titrating hormone replacement, and then every 6-12 months once stable, is essential to prevent both overtreatment and undertreatment 5
• Overtreatment with levothyroxine can increase risk for osteoporosis, fractures, abnormal cardiac output, and ventricular hypertrophy 4, 5
• Appropriate dosing is critical: for patients <70 years without cardiac disease, the full replacement dose is approximately 1.6 mcg/kg/day; for patients >70 years or with cardiac disease, start with 25-50 mcg/day and titrate gradually 5

Clinical Implications

• Patients should be informed that weight gain can occur with levothyroxine treatment, contrary to expectations 1, 2
• Weight management should be addressed as part of comprehensive care for patients with hypothyroidism 3
• The goal of treatment should be to normalize TSH levels rather than to achieve weight loss 5, 3
• Careful dose adjustment and regular monitoring are essential to minimize weight-related side effects 5