What Are Some Side Effects of the Thyroid Drug, Synthroid?

The most common side effect of Synthroid, a drug used to treat hypothyroidism, is temporary hair loss within the first few months of treatment, according to WedMD. Other potential side effects include skin rash, changes in appetite, diarrhea, headache and nausea, as reported by AbbVie US LLC, the drug's manufacturer,

Chest pain, increased or irregular heartbeat, or signs of an allergic reaction, such as swelling of the face, neck, mouth, or tongue or trouble breathing, require immediate medical attention, according to CVS Pharmacy. Patients with diabetes may notice changes in blood sugar levels that should be monitored by a doctor.