Levothyroxine is a synthetic form of thyroid hormone used to treat hypothyroidism (underactive thyroid). According to the U.S. Food and Drug Administration (FDA) and Mayo Clinic, levothyroxine is generally safe when taken as prescribed, but side effects may occur, especially if the dose is too high. Understanding these potential side effects helps patients recognize when to seek medical advice.

20 Side Effects of Levothyroxine

Common Side Effects

Headache
Nervousness or anxiety
Irritability
Sweating more than usual
Heat intolerance
Increased appetite
Weight loss
Diarrhea
Insomnia or difficulty sleeping
Tremors

Less Common but Reported Side Effects

Irregular heartbeat (arrhythmia)
Rapid heart rate (tachycardia)
Chest pain or discomfort
Shortness of breath
Muscle weakness
Hair loss (temporary, especially in children at the start of treatment)
Menstrual irregularities
Vomiting or stomach cramps
Nervous system excitability (restlessness, hyperactivity)
Seizures (rare, but possible in extreme cases of overdose or improper dosing)

When to Contact a Doctor

Patients should seek immediate medical care if they experience severe chest pain, irregular heartbeat, shortness of breath, or seizures. These could indicate an overdose or sensitivity to the medication.

Safe Use of Levothyroxine

Always take it exactly as prescribed, on an empty stomach.
Avoid switching brands without consulting your doctor, as potency may differ slightly.
Regular blood tests help ensure the dose remains correct.