Possible Synthroid Side Effects
Synthroid can have side effects. Synthroid side effects may be caused by under-replacement (too little of the active ingredient levothyroxine) or over-replacement (too much levothyroxine). If you are not getting
enough levothyroxine, you may continue to experience hypothyroidism symptoms. If you are getting too much levothyroxine, you'll
experience hyperthyroidism (overactive thyroid) symptoms. Notify your doctor if
you experience any of the symptoms on this page. It may be helpful to use a
treatment tracker to record when you experience symptoms, and whether or
not they are still occurring.
Partial hair loss may occur rarely during the first few months of Synthroid therapy,
but this is usually temporary.
Side effects* of over-replacement can include:
- Rapid or irregular heartbeat
- Chest pain or shortness of breath
- Muscle weakness
- Change in appetite
- Weight loss
- Frequent bowel movements
- Excessive sweating
- Heat intolerance
- Impaired fertility
- Decreased bone density
- Changes in menstrual periods
- Other unusual medical events
Always talk to your doctor if you have
questions or concerns about your hypothyroidism treatment.
1. Kronenberg HM. Williams Textbook of Endocrinology.
11th ed. Philadelphia, PA: Saunders Elsevier; 2008.
2. American Thyroid Association. Hypothyroidism. A booklet for patients and their families. http://www.thyroid.org/wp-content/uploads/patients/brochures/Hypothyroidism_web_booklet.pdf. Published 2013. Accessed January 23, 2014.
3. Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocr Pract. 2012;18(6):988-1028.