Does Synthroid Cause Weight Gain?

Synthroid (levothyroxine) typically does not cause weight gain and may actually lead to modest weight loss. This synthetic thyroid hormone works by replacing or supplementing your body's natural thyroid hormone, which regulates metabolism. When thyroid levels normalize, many patients experience a 5-10% reduction in body weight over 6-12 months. However, some people may notice initial water retention or appetite changes during the first few weeks of treatment. If you experience unexpected weight gain while taking Synthroid, it could indicate your dose needs adjustment or that underlying thyroid issues aren't fully corrected. Weight changes are often related to the thyroid condition itself rather than the medication.