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

Levothyroxine replaces a hormone your thyroid gland is not making enough of. When taken at the right dose, most patients tolerate it well and experience few or no side effects. The side effects that do occur are almost always caused by too high a dose — meaning your body is getting more thyroid hormone than it needs. Here's what to watch for.

The Most Important Thing to Know About Levothyroxine Side Effects

Levothyroxine carries a

boxed warning (the FDA's strongest warning): levothyroxine should NOT be used for weight loss or obesity treatment. Taking more than your prescribed dose — especially in euthyroid patients — can cause serious or life-threatening cardiovascular effects, including heart arrhythmias.

Common Side Effects (Signs of Too-High Dose)

If your levothyroxine dose is slightly too high, you may experience symptoms that resemble hyperthyroidism (an overactive thyroid). These include:

Heart palpitations or rapid heartbeat (tachycardia)

Nervousness, irritability, anxiety

Insomnia or difficulty sleeping

Tremors (shaking hands)

Increased sweating or heat intolerance

Weight loss or increased appetite

Diarrhea or more frequent bowel movements

Headache

Muscle weakness

Menstrual irregularities

Temporary hair loss can also occur early in treatment as the body adjusts, but this typically resolves within a few months.

Serious Side Effects: When to Call Your Doctor Immediately

Seek immediate medical attention if you experience:

Chest pain or chest tightness — could indicate cardiac effects of excessive thyroid hormone

Severe irregular heartbeat (arrhythmia) — serious cardiovascular risk with overdose

Shortness of breath — may signal cardiac involvement

Seizures — rare but reported, especially at initiation of therapy

Signs of adrenal crisis — in patients with undiagnosed adrenal insufficiency, starting levothyroxine can precipitate adrenal crisis (extreme fatigue, dizziness, nausea, low blood pressure)

Long-Term Side Effects to Monitor

Bone density loss: Long-term levothyroxine at doses that suppress TSH can decrease bone mineral density, increasing osteoporosis risk — particularly in postmenopausal women. Regular bone density screening is recommended for patients on suppressive doses.

Cardiovascular effects: Chronic overtreatment increases heart rate and cardiac workload. Elderly patients and those with pre-existing heart disease are most vulnerable to cardiac complications from excess thyroid hormone.

Side Effects in Children

In children, overtreatment with levothyroxine can cause pseudotumor cerebri (increased intracranial pressure), slipped capital femoral epiphysis, and premature closure of the epiphyseal growth plates — which can compromise adult height if untreated. Regular dose review and growth monitoring are essential in pediatric patients.

Food and Supplement Interactions That Cause Side Effects

Reduced absorption of levothyroxine — from taking it too close to food, coffee, or supplements — can mimic under-treatment (hypothyroid symptoms). Always take levothyroxine on an empty stomach, 30-60 minutes before eating. Separate from calcium and iron supplements by at least 4 hours.

For a full list of drug and food interactions, see our guide: Levothyroxine Drug Interactions: What to Avoid and What to Tell Your Doctor.

For a general overview of the medication, see What Is Levothyroxine? Uses, Dosage, and What You Need to Know in 2026.