The short answer: A levothyroxine dose that is too high causes symptoms of an overactive thyroid: a racing or pounding heart, anxiety, insomnia, tremor, sweating, heat intolerance, diarrhea, and weight loss. The FDA label for Synthroid (revised February 2024) lists these as signs of overdosage. A blood test showing a low or suppressed TSH confirms it.

What does too much levothyroxine feel like?

Levothyroxine replaces the hormone your thyroid no longer makes. When the dose is more than your body needs, you develop the symptoms of hyperthyroidism, even though your own thyroid is underactive. Doctors call this overtreatment or iatrogenic hyperthyroidism.

The FDA prescribing information says the side effects of levothyroxine are "primarily those of hyperthyroidism due to therapeutic overdosage." The label groups them by body system:

  • Heart: palpitations, fast heart rate, irregular rhythm, higher pulse and blood pressure, and in serious cases angina, heart failure, or heart attack.
  • Brain and mood: headache, nervousness, anxiety, irritability, mood swings, hyperactivity, and insomnia.
  • Muscles: tremor, muscle weakness, and muscle spasms.
  • Whole body: fatigue, increased appetite, weight loss, heat intolerance, fever, and heavy sweating.
  • Gut: diarrhea, vomiting, and abdominal cramps.
  • Skin and hair: hair loss, flushing, and rash.
  • Reproductive: menstrual changes and reduced fertility.
  • Breathing: shortness of breath.

The anxiety and sleep problems are easy to mistake for stress. If they began after a dose change, consider the medication first. Our guide to thyroid-related anxiety and panic attacks covers how the two overlap. If sleep is the main problem, insomnia.md explains how insomnia is assessed once a thyroid cause is ruled out.

Fatigue is on the list too, which surprises many people. Feeling tired does not always mean the dose is too low. A blood test is the only way to tell which direction the problem goes.

Which lab results show the dose is too high?

For most adults with primary hypothyroidism, the FDA label says "the general aim of therapy is to normalize the serum TSH level." TSH is the pituitary signal that rises when thyroid hormone is low and falls when it is high. So on levothyroxine:

  • Low or suppressed TSH usually means you are getting more hormone than you need.
  • High free T4 often appears with it, though free T4 can sit near the top of the normal range.
  • High TSH points the other way, toward too little.

How low matters. A Scottish cohort of 17,684 people on long-term thyroxine, published in the Journal of Clinical Endocrinology and Metabolism in 2010, split patients by TSH. Those with a suppressed TSH (0.03 mU/L or lower) had more heart disease, rhythm problems, and fractures. Those with a low but not suppressed TSH (0.04 to 0.4 mU/L) did not show a higher risk.

A low TSH is not always overtreatment. Some thyroid cancer patients are kept suppressed on purpose. MedlinePlus notes that serious non-thyroid illness can lower TSH for a short time. The FDA label also tells patients to stop biotin supplements at least 2 days before thyroid tests, because biotin can distort results. For a walk-through of each value, see thyroid blood tests explained.

Why would a dose that worked become too high?

Your dose is set for your body and your routine at one point in time. When something changes, the same tablet can deliver more hormone than you need. Common reasons, each tied to what the label or studies describe:

  • Weight loss. The label bases full replacement on body weight (1.6 mcg/kg/day). In a 2009 study of adults 65 and older, every 10 kg of lower weight raised the odds of a low TSH by 65%.
  • A different tablet. MedlinePlus warns not to switch brands without your doctor, because each brand contains a slightly different amount of medication.
  • Stopping something that blocked absorption. The label lists calcium carbonate, iron, antacids, proton pump inhibitors, and bile acid binders as drugs that reduce absorption. Stop one, and more levothyroxine gets in.
  • After pregnancy. Doses usually rise during pregnancy. The label says to return to the pre-pregnancy dose right after delivery and recheck TSH 4 to 8 weeks postpartum.
  • Dosing errors. Taking a dose twice, mixing up strengths after a refill, or a new pill that looks different.
  • Aging. In that 2009 study, 41% of older thyroid hormone users had a low TSH, more than had a high one.

What are the long-term risks of overtreatment?

Mild symptoms are the short-term problem. The bigger concern is what years of excess hormone do to the heart and bones. The FDA label names both.

Heart rhythm. The label warns that overtreatment can raise heart rate and wall thickness and can trigger angina or arrhythmias. The risk is highest in older adults and people with heart disease. It states that "atrial fibrillation is the most common of the arrhythmias observed with levothyroxine overtreatment in the elderly."

Bone loss. The label says over-replacement can increase bone breakdown and lower bone mineral density, "particularly in post-menopausal women." A 2011 BMJ study of 213,511 adults aged 70 or older in Ontario found fracture risk rose with cumulative levothyroxine dose. The authors called for ongoing dose monitoring to avoid overtreatment.

These are reasons to check your TSH on schedule, not reasons to stop your medication. Untreated hypothyroidism carries its own risks. The 2014 American Thyroid Association guideline concluded that levothyroxine remains the standard of care.

When should you call a doctor or get emergency care?

Call your prescriber within a few days if you notice new palpitations, shakiness, sweating, insomnia, anxiety, diarrhea, or unplanned weight loss. This matters most after a dose change, a pharmacy switch, or stopping a supplement.

Call your doctor right away for chest pain or a rapid or irregular heartbeat. MedlinePlus lists these among the symptoms to report immediately.

Call 911 if someone collapses, has a seizure, has trouble breathing, or cannot be woken. For a suspected overdose, such as a child swallowing tablets or someone taking far more than prescribed, MedlinePlus advises calling Poison Control at 1-800-222-1222. The FDA label notes that overdose symptoms may not appear until several days after ingestion.

What happens after you report symptoms?

Do not cut, split, or skip doses on your own. The label says to reduce or pause the dose if overdosage signs occur, but that is a clinician decision. Here is the usual sequence, based on the FDA label:

  1. Before the test: stop biotin at least 2 days ahead. Bring a list of every supplement and medication, including anything you started or stopped.
  2. Test: your clinician orders TSH, often with free T4.
  3. Decision: if TSH is low and symptoms fit, your clinician may lower the dose. People at risk of atrial fibrillation get slower, smaller adjustments.
  4. Weeks 4 to 6: the label says peak effect of a dose change may take 4 to 6 weeks.
  5. Weeks 6 to 8: TSH is rechecked 6 to 8 weeks after any dose change.
  6. Once stable: the label recommends checking every 6 to 12 months, and again whenever your health changes.

For how to take the tablet each day, see our levothyroxine guide.

The bottom line

A racing heart, anxiety, poor sleep, tremor, sweating, and weight loss on levothyroxine can mean the dose is too high. A low or suppressed TSH confirms it. Weight loss, a tablet switch, stopping a supplement, or giving birth can all tip a stable dose into overtreatment. Over years, the FDA label links overtreatment to atrial fibrillation and bone loss, especially in older adults and postmenopausal women. Report symptoms, get a TSH test, and let your clinician adjust the dose.

Last updated: November 2026. This article is for informational purposes only and does not constitute medical advice. Do not change your levothyroxine dose without talking to the clinician who prescribes it.