Levothyroxine Side Effects: What's Normal, What's Overtreatment, and When to Worry

You started levothyroxine to treat an underactive thyroid, and now your heart is racing, you can't sleep, or you've noticed your hair thinning in the shower. It's unsettling to feel worse on a medication that's supposed to help. The good news: most side effects from levothyroxine are a sign that your dose simply needs adjusting, not that anything is permanently wrong.

Here's the key idea to hold onto. Levothyroxine is a replacement for a hormone your body already makes, so when the dose is right, it usually causes very few problems. Most troubling side effects come from getting a little too much, which pushes your body into a state that mimics an overactive thyroid. Understanding the difference helps you know what to watch for and when to call your clinician.

What are the side effects of levothyroxine?

Levothyroxine replaces the thyroid hormone (T4) your body is low on. When the dose matches what your body needs, side effects are usually minimal. When the dose is even slightly too high, you start to feel the symptoms of an overactive thyroid, because there's now more hormone circulating than your body wants.

According to the Mayo Clinic, the most common side effects reflect this excess and include:

• Fast or pounding heart rate
• Trouble sleeping (insomnia)
• Tremors, nervousness, or feeling on edge
• Heat intolerance or sweating more than usual
• Increased appetite alongside unexpected weight loss
• Diarrhea
• Temporary hair loss, often in the first few months of treatment

Why does levothyroxine make my hair fall out?

Hair loss is one of the most distressing side effects, partly because an underactive thyroid can itself cause thinning hair, so it feels like the medication isn't working. In reality, the Mayo Clinic notes that temporary hair loss can occur in the first few months of levothyroxine therapy as your body adjusts to the new hormone level.

For most people this shedding settles as treatment stabilizes. If hair loss is severe, keeps getting worse, or continues well beyond the early months, mention it to your clinician, because it can occasionally be a clue that your dose needs fine-tuning or that something else is going on.

Is levothyroxine overtreatment common?

Yes, and this is the part many patients don't hear about. Taking more levothyroxine than you need (overtreatment) is far more common than taking too little. A 2023 population-based Rhineland Study found that 18% of levothyroxine users were overtreated, compared with only 4% who were undertreated. In older adults, suppressed thyroid levels indicating overtreatment are especially common.

Overtreatment matters because the extra hormone strains the heart and bones over time. The single most common heart rhythm problem linked to too much levothyroxine is atrial fibrillation, an irregular heartbeat. This is why doctors check your TSH (a blood test) periodically and adjust your dose rather than setting it once and forgetting it.

What are the serious risks of levothyroxine?

Properly dosed levothyroxine is considered very safe, and the risk concentrates in overtreatment rather than the medication itself. Reassuringly, a 2026 systematic review in the British Journal of Clinical Pharmacology found that well-designed, blinded, placebo-controlled trials did not show statistically significant cardiac, neuropsychiatric, or musculoskeletal harm from levothyroxine. The worrying associations came mostly from observational studies that are prone to bias.

That said, chronic excess dosing does carry real risks, especially for older adults and postmenopausal women. A BMJ nested case-control study of 213,511 adults aged 70 and older found that current levothyroxine use carried an adjusted odds ratio of 1.88 (95% CI 1.71 to 2.05) for fractures compared with past use, and high cumulative doses raised fracture risk 3.45-fold compared with low doses. In rare cases of severe overdose, FDA labeling notes that heart attack, cerebral embolism, shock, coma, and death have been reported. These extremes are uncommon and tied to far-too-high amounts, not standard therapy.

Can I take levothyroxine for weight loss?

No. This is a common misconception, partly because excess thyroid hormone can cause weight loss. The Mayo Clinic is explicit that levothyroxine should not be used for weight loss or obesity. It is ineffective for that purpose in people with a normal thyroid, and taking larger-than-needed amounts to lose weight is dangerous, pushing you toward the heart and bone risks of overtreatment.

If you have a genuinely underactive thyroid, treating it can help your metabolism return to normal, but the goal is to restore balance, not to overshoot it for faster weight loss.

When should I see a doctor about levothyroxine side effects?

Most side effects are manageable with a dose adjustment, but some symptoms need prompt attention. Call your clinician if you develop a fast, pounding, or irregular heartbeat, chest pain, or significant shortness of breath, as these can signal overtreatment or a heart rhythm problem. Severe or worsening symptoms warrant urgent care, and chest pain with shortness of breath should be treated as an emergency.

For milder but persistent issues, like ongoing insomnia, tremors, heat intolerance, diarrhea, or hair loss that isn't settling, it's worth a conversation and likely a TSH blood test. Never stop or change your dose on your own, since both too much and too little thyroid hormone cause problems. The right move is to work with your clinician to dial in the dose that keeps you in the normal range.