Levothyroxine can cause weight loss, especially when it is used to treat hypothyroidism. Around half of people who take levothyroxine (Synthroid) may lose about 4 to 9 pounds (2 to 4 kg), or sometimes a little more.
Side effects like weight loss are usually more common when the dose of levothyroxine is too high. This is why regular thyroid tests are important at the start of treatment, so the dose can be adjusted properly and significant side effects can be avoided.
Unwanted side effects, including weight loss, often stop once the medication is adjusted to the right dose. It is also important to watch for other symptoms that may suggest the dose is too high.
Talk to your primary care provider about any unexplained weight loss, or consider seeing an endocrinologist to review your treatment.