Synthroid Side Effects: What to Expect and When to Call Your Doctor

Synthroid (levothyroxine) is generally well-tolerated when taken at the correct dose. In fact, most of its side effects are caused by getting the dose wrong — either too much or too little — rather than by the medication itself. Understanding what symptoms are normal, what's a sign of dose adjustment, and what requires immediate medical attention can help you manage your treatment safely.

The Boxed Warning: Do Not Use for Weight Loss

Synthroid's prescribing information includes a boxed warning (the FDA's most serious type): Synthroid should NOT be used for the treatment of obesity or for weight loss. In patients with normal thyroid function, doses within the normal hormonal range are ineffective for weight reduction. Higher doses can produce serious or even life-threatening toxicity, particularly when combined with sympathomimetic agents (like stimulant weight-loss drugs).

Why Most Side Effects Are Signs of Overdose

Levothyroxine replaces a hormone your body already produces naturally. When you take exactly the right amount, you feel normal — just like your thyroid were working correctly. Side effects typically occur when:

Dose is too high (hyperthyroid symptoms): rapid heartbeat, weight loss, heat intolerance, sweating, anxiety, insomnia

Dose is too low (hypothyroid symptoms return): fatigue, weight gain, cold sensitivity, brain fog, dry skin, constipation

Regular TSH monitoring every 6–12 months (and at 6–8 weeks after any dose change) is how your doctor keeps your dose in the right range.

Common Side Effects of Synthroid

The following side effects are associated primarily with over-replacement (dose too high):

Rapid or irregular heartbeat (palpitations)

Weight loss and increased appetite

Heat intolerance and excessive sweating

Nervousness, anxiety, or irritability

Insomnia or difficulty sleeping

Diarrhea or frequent bowel movements

Tremors (shakiness)

Muscle weakness

Menstrual irregularities

Temporary hair loss (usually in the first few months of starting treatment — typically resolves on its own)

Serious Side Effects: When to Seek Immediate Help

Call 911 or go to the emergency room if you experience:

Chest pain or tightness — can indicate angina or cardiac strain from excess thyroid hormone

Rapid, pounding, or irregular heartbeat — particularly in elderly patients or those with underlying heart disease, where atrial fibrillation is a risk

Shortness of breath — especially with exertion

Signs of adrenal crisis (severe weakness, confusion, vomiting, low blood pressure) — can occur in patients with undiagnosed adrenal insufficiency who start levothyroxine

Seizures — rarely reported with initiation of levothyroxine therapy

Special Concerns: Bone Loss and Cardiovascular Risk

Over-replacement with levothyroxine — even mildly suppressed TSH — can cause decreased bone mineral density over time, increasing the risk of osteoporosis and fractures. This is particularly concerning for post-menopausal women. Your doctor should monitor for this and aim for the minimum effective dose.

In elderly patients and those with coronary artery disease, excessive levothyroxine can increase heart rate, cardiac wall thickness, and contractility, and may precipitate angina or arrhythmias. Dose should always start low in these populations.

Side Effects in Children on Synthroid

Children receiving levothyroxine require careful monitoring. Overtreatment can cause craniosynostosis (premature skull closure) in infants, early closure of growth plates leading to reduced adult height, and pseudotumor cerebri (increased intracranial pressure). Undertreatment can impair intellectual development and linear growth.

When Should You Call Your Doctor?

Contact your doctor (not an ER, unless symptoms are severe) if you notice:

Persistent palpitations or racing heart that don't go away

New or worsening anxiety, tremors, or insomnia after dose changes

Unexplained weight loss or gain

Significant hair loss persisting beyond 3–4 months

Returning symptoms of hypothyroidism (fatigue, constipation, cold intolerance, weight gain)

If you're struggling to find your medication, medfinder can help locate Synthroid at a nearby pharmacy. Also review our guide on Synthroid drug interactions to ensure nothing you're taking is affecting your dose.