Accidentally Took Double Dose of Levothyroxine (Synthroid): What to Do

If you accidentally took two levothyroxine pills, it is unlikely to cause problems. Still, call your pharmacist, your doctor or Poison Help at 1-800-222-1222 (1-844-764-7669 in Canada) for advice as soon as you can. The NHS says an extra dose "is unlikely to cause problems, but it's important to get medical advice if you take too much as it can sometimes be dangerous." Levothyroxine (Synthroid) has a long half-life of 6 to 7 days. A single extra dose creates a small proportional increase in your overall hormone levels. Do not skip or change your next dose on your own. The Levoxyl patient information says not to change your dose "unless your healthcare provider tells you to."

Medical Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice. Consult your doctor or pharmacist for advice specific to your medications.

Here is why one extra tablet is usually not dangerous.

Why a double dose of levothyroxine is usually a non-issue

Levothyroxine replaces the thyroid hormone (T4) your body is not making enough of on its own. Your body already maintains a large, stable pool of T4 that builds up over weeks of daily dosing. One extra pill adds a small amount to that pool.

With a half-life of 6 to 7 days, your body holds more than a week's worth of daily doses at any given time. Adding one extra daily dose to that pool is like adding one more cup of water to a pitcher that already holds more than seven. The proportional increase is small.

Research on much larger single doses points the same way. In a small study published in the Journal of Clinical Endocrinology & Metabolism in 1997, 12 people with hypothyroidism took 7 times their usual daily dose once a week instead of a pill every day. The researchers found "no evidence of treatment toxicity, including cardiac toxicity," although TSH ran higher than with daily dosing. That is a study result, not a dosing plan, so do not change how you take your own dose without your prescriber.

Levothyroxine doses are measured in micrograms (mcg), not milligrams. Synthroid tablets come in 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, and 300 mcg. There is no fixed "maximum dose" because the right dose is individualized based on your TSH levels.

Whichever row you are in, the first step is the same: call your pharmacist, your doctor or Poison Help.

What to do right now

1. Stay calm. One extra tablet is unlikely to cause problems.
2. Call before your next dose. Your pharmacist, your prescriber or Poison Help (1-800-222-1222; in Canada, 1-844-764-7669, or 1-800-463-5060 in Quebec) can tell you whether to take your next dose as usual. Do not skip it or change it on your own. Poison Help's expert guidance is "free, confidential, and available 24 hours a day."
3. Take it with water as usual. If you have not eaten yet, wait 30 to 60 minutes before eating and a full 60 minutes before coffee to avoid absorption issues.
4. Write down the time and amount you took. Have it in front of you when you call, and mention it at your next appointment.
5. Do not make yourself vomit. MedlinePlus says not to unless poison control or a health care professional tells you to.

Symptoms to watch for

Mild symptoms (unlikely but possible)

Most people will not notice any symptoms from a single double dose. Some may notice:

• Mild anxiety or nervousness
• Slight increase in heart rate
• Feeling warmer than usual
• Mild headache

These would be signs that your thyroid hormone levels are temporarily higher than your usual range. The Synthroid label warns that overdose symptoms "may not appear until several days after ingestion," so watch for them over the next several days and call your doctor or pharmacist right away if any show up.

Serious symptoms (call your doctor right away)

MedlinePlus says to "call your doctor immediately or get emergency medical treatment" for symptoms such as:

• Chest pain
• Rapid or irregular heartbeat or pulse
• Uncontrollable shaking
• Shortness of breath
• Excessive sweating

These symptoms are unlikely from a single double dose at typical prescribed levels, but they are more of a concern if you take a higher dose or if you have underlying heart conditions.

Emergency symptoms (call 911)

MedlinePlus says to call 911 right away if someone:

• Has trouble breathing
• Has collapsed or fainted
• Has had a seizure
• Cannot be woken up

When to call your doctor or Poison Control

Any accidental double dose is worth a call to your pharmacist or Poison Help, because the NHS advises getting medical advice when you take too much. Contact your doctor as well if:

• You took more than one extra dose (three or more pills in a day)
• You have a heart condition (atrial fibrillation, coronary artery disease, angina). Excess thyroid hormone can strain the heart, and people with existing heart conditions are more sensitive.
• You are elderly. Older adults are more sensitive to changes in thyroid hormone levels.
• You are on a high dose and are concerned
• You notice any serious symptoms listed above

Contact numbers:

• Poison Control: 1-800-222-1222 (free, 24/7)
• Poison centres in Canada: 1-844-764-7669 (toll-free), or 1-800-463-5060 in Quebec
• Your pharmacist: Quick, accessible guidance
• 911: For emergencies

If your thyroid medicine is not plain levothyroxine, or you are in a higher-risk group

Most of this guide assumes one extra levothyroxine tablet in a healthy adult. A few situations change the picture. None of them turn one extra pill into an emergency on its own, but each one lowers the bar for picking up the phone.

Cytomel (liothyronine), Armour Thyroid, or NP Thyroid

Levothyroxine is T4, the slow-acting hormone. Cytomel is liothyronine, which is T3, and thyroid tablets made from porcine thyroid gland, such as Armour Thyroid and NP Thyroid, contain both. The Armour Thyroid label states that the tablets "provide 38 mcg levothyroxine (T4) and 9 mcg liothyronine (T3) per grain of thyroid" and that "T3 liothyronine is approximately four times as potent as T4 levothyroxine on a microgram for microgram basis." The Cytomel label puts its biological half-life at "about 2-1/2 days," against 6 to 7 days for levothyroxine, and notes that "CYTOMEL has a rapid onset of action." In plain terms, T3 works faster and is stronger per microgram than levothyroxine, even when the number on the bottle looks similar, although the Cytomel label also says overdose symptoms "may not appear until several days after ingestion." The same first call applies, and with a T3 product it matters more: call Poison Help at 1-800-222-1222 (1-844-764-7669 in Canada) the same day rather than waiting to see how you feel.

Pregnancy

Thyroid hormone needs often rise in pregnancy. The Synthroid label dosing table says a pre-pregnancy dosage "may increase during pregnancy" and calls for TSH checks "every 4 weeks until a stable dose is reached." Tell your prescriber about an extra tablet the same day, and do not adjust your dose on your own.

Age 65 and over, or any heart condition

The Synthroid label is direct about who feels excess thyroid hormone first: "Atrial arrhythmias can occur in elderly patients. Atrial fibrillation is the most common of the arrhythmias observed with levothyroxine overtreatment in the elderly." If you are over 65 or have coronary artery disease, angina, or a history of irregular heartbeat, treat any new palpitations or chest discomfort after a double dose as a reason to call your doctor right away, and call 911 for trouble breathing, chest pain or fainting.

You took the second tablet at night

Taking your usual morning dose and then a second tablet in the evening is still one extra dose in one day. Call your pharmacist or Poison Help, as described above, and ask whether to take your next morning dose as usual. The Synthroid label lists "nervousness, irritability, insomnia" among the effects of too much hormone, so a restless night can be the first thing you notice.

Three or more tablets, or doubling for several days

This is where the picture changes. The Synthroid label says the drug "has a narrow therapeutic index," that "the signs and symptoms of overdosage are those of hyperthyroidism," that "confusion and disorientation may occur," and that in serious cases "cerebral embolism, shock, coma, and death have been reported." Three or more tablets in one day is beyond a routine double and is a Poison Help call now, not a wait-and-see. Doubling for several days because two bottles looked alike, or a refill came in a different strength, is a different problem again: that is a prescriber call today, and bring both bottles so the strengths can be compared.

How to prevent accidental double dosing

Levothyroxine has a unique challenge: it is usually the first thing you take in the morning, on an empty stomach, often before you are fully awake. That groggy "did I already take this?" moment is a common cause of double dosing. A few solutions:

Track every dose with a medication reminder app

Regular phone alarms tell you when to take a pill, but they do not record whether you actually did. A dedicated tracking app logs every confirmed dose, so you always have a clear answer to "did I already take this?"

Pillo tracks every dose you confirm, uses persistent alarms that will not stop until you respond, and keeps a history of exactly what you took and when. Because levothyroxine is usually the first pill of the day (and often needs to be taken 30 to 60 minutes before food and a full hour before coffee), having a clear confirmation record eliminates the early-morning guesswork.

Use a weekly pill organizer

A 7-day organizer is especially useful for levothyroxine. Set it up on Sunday, and each morning you have instant visual proof: compartment empty means you already took it.

Take it at the same time every day

The best time to take levothyroxine is on an empty stomach, at least 30 to 60 minutes before breakfast. Picking a consistent wake-up routine and anchoring your dose to it reduces confusion. Some people even keep the pill bottle on their nightstand and take it the moment their alarm goes off.

Mind the coffee gap

Many people take levothyroxine and then want coffee immediately. Coffee can reduce levothyroxine absorption, so you need to wait at least 60 minutes. This waiting period sometimes leads people to forget whether they took the pill at all. Tracking your dose when you take it solves this.

Frequently asked questions

Is a double dose of levothyroxine dangerous?

For most people, no. Levothyroxine has a half-life of 6 to 7 days, meaning your body already holds more than a week's worth of daily doses. One extra dose creates a very small proportional increase. The NHS still advises getting medical advice when you take too much, as it can sometimes be dangerous, so call your pharmacist or Poison Help. The risk is higher for elderly patients and people with heart conditions.

Can a double dose of levothyroxine cause thyrotoxicosis?

Thyrotoxicosis (excess thyroid hormone symptoms like rapid heartbeat, anxiety, tremor, sweating) is very unlikely from a single accidental double dose. If symptoms like these appear, even several days later, call your doctor right away.

Should I skip my next dose of levothyroxine after doubling up?

Not on your own. Ask your pharmacist or prescriber first. The Levoxyl patient information says not to change your dose "unless your healthcare provider tells you to." MedlinePlus says to call Poison Control at 1-800-222-1222 in case of overdose, and the NHS says to speak to a pharmacist as soon as possible if you've taken more than your prescribed dose. For guidance on missed doses in general, see our missed dose of levothyroxine guide or the broader missed dose of thyroid medication guide.

How long until the extra levothyroxine wears off?

The Synthroid label puts levothyroxine's half-life at 6 to 7 days when thyroid levels are in range (9 to 10 days when they are low), so about half or more of the extra amount is still in your body a week later, and the rest keeps clearing over the following weeks. It also notes that overdose symptoms "may not appear until several days after ingestion," so call your doctor or pharmacist right away if you notice any.

Can I take levothyroxine with coffee?

Coffee can reduce levothyroxine absorption. Wait at least 60 minutes after taking levothyroxine before drinking coffee, as a 2021 review of food interactions advises. This is true whether you accidentally doubled your dose or took it normally.

What if I am not sure whether I already took my levothyroxine?

If you genuinely cannot remember, ask your pharmacist what to do before you take another tablet or skip one. Read our guide on what to do when you cannot remember if you took your medication for more on this situation.

Is it different if I take Armour Thyroid or Cytomel instead of levothyroxine?

Yes. Cytomel is liothyronine (T3), and Armour Thyroid and NP Thyroid are made from porcine thyroid gland and contain both T4 and T3. The Armour Thyroid label describes T3 as approximately four times as potent as T4 microgram for microgram, and the Cytomel label notes a rapid onset of action with a biological half-life of about 2-1/2 days. The same plan applies, but call Poison Help the same day if you take a T3-containing product, even if you feel fine, because overdose symptoms can take several days to appear.

Will a double dose mess up my next TSH test?

One extra tablet is a small change against a 6 to 7 day half-life. Still, the American Association of Clinical Endocrinologists guidelines call serum TSH, in most outpatient situations, the most sensitive test for detecting mild thyroid hormone excess or deficiency, so if you have a blood test within a week or two of the mix-up, tell your prescriber the date and amount so they can read the result in context.

Related guides

• Missed a dose of levothyroxine? Here is what to do
• Best time to take levothyroxine
• Coffee and medication: how long to wait
• Missed a dose of thyroid medication

If remembering doses is the hard part, a pill reminder app that keeps alarming until you respond can take that job off your plate.

Medical Disclaimer: This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. If you think you may have a medical emergency, call 911 or your local emergency number immediately.

Reviewed sources: FDA Synthroid Label, Weekly levothyroxine dosing study, Poison Control

Sources

1. Peer-reviewed study (DOI: 10.1210/jcem.82.3.3830). study published in the Journal of Clinical Endocrinology & Metabolism
2. National Capital Poison Center (Poison Control). Double dosing: what to do if you take too much
3. FDA Prescribing Information, Synthroid (DailyMed). half-life of 6 to 7 days (Table 10); tablet strengths; narrow therapeutic index; atrial fibrillation in the elderly; overdosage
4. FDA Prescribing Information, Cytomel (DailyMed). biological half-life is about 2-1/2 days
5. Armour Thyroid labeling (DailyMed; unapproved drug, labeling not approved by FDA). 38 mcg levothyroxine (T4) and 9 mcg liothyronine (T3) per grain
6. NP Thyroid labeling (DailyMed; unapproved drug, labeling not approved by FDA). 38 mcg levothyroxine (T4) and 9 mcg liothyronine (T3) per grain
7. Baskin HJ et al., AACE Medical Guidelines, Endocr Pract 2002;8(6):457-69. PubMed 15260011
8. NHS, Levothyroxine. If you take too much levothyroxine
9. FDA-approved Patient Information, Levoxyl (DailyMed). How should I take LEVOXYL?
10. MedlinePlus, Levothyroxine. In case of emergency/overdose
11. MedlinePlus, Poisoning first aid. Do not make a person throw up unless told to
12. Health Canada, March 20, 2023. 1-844 POISON-X (1-844-764-7669); Quebec 1-800-463-5060

Reviewed under our Medical Review Policy.