What it is

Levothyroxine is T4, the same hormone your thyroid makes. Your body turns it into T3, the active form, just as it does with your own T4.^2,3 The tablet has a score line, so it can be split into halves. Your doctor prescribes the dose based on your blood test.

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How to take it

• Once a day at bedtime, at least 3 hours after your last meal. Food lowers how much of the tablet you absorb. In one study, people absorbed about 80% of the tablet on an empty stomach, but only 64% with food.³
• At the same time every day, so your body absorbs the same amount each day and your blood tests give a true picture.³
• With about half a glass of water.¹
• Take the amount written on your label. If your label says part of a tablet, split it along the score line.
• On the morning of a blood test, there is no need to skip a dose. Your bedtime dose is many hours before the test, so it does not affect the result.

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Keep it apart from food, drinks and other medicines

These things stick to the tablet in your gut, or change how it works, so less of it reaches your blood.⁴

• Coffee: at least 1 hour apart.
• Calcium, iron, and antacids (indigestion medicines): at least 4 hours apart, taking your levothyroxine first.^2,4
• Soy foods and high-fibre foods or supplements: at least 4 hours apart.⁴

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Always the same product

• Take only the product we give you. Even with the same name and strength, products from different makers can give different amounts in your blood.^2,4
• Be careful at pharmacies. A pharmacy may give you another maker’s tablets without telling you. The same brand may also contain different ingredients in different countries.
• If you ever have to change product, tell us. You will need a blood test 6 weeks later, because the new product may give a different level.⁴

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If you miss a dose

Skip the missed dose, and take your next dose at bedtime as usual. Never take a double dose to make up. Missing one dose is not a worry, because the medicine stays in your body for about a week.² If you miss 2 or more doses in a row, please tell us. Missed doses can change your blood test result, and we need to know this when we read it.

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If you stop

Levothyroxine stays in your body for about a week, so your level falls slowly.² A missed day does not leave you without hormone. If you stop for longer, it takes about 6 weeks for your body to settle at its own level again. This is why we wait 6 weeks before testing after you stop.³ Please talk to us before you stop, unless you have one of the symptoms below.

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Please stop

• Biotin, for 3 days before each blood test. Biotin is found in many hair, skin and nail vitamins and in multivitamins. It can make your thyroid results look wrong.²
• Weight-loss or appetite-suppressing products. Levothyroxine is not a weight-loss medicine, and taken together with these products it can cause serious side effects.²
• Hormone or thyroid products from other sources. Their quality varies, and your doctor cannot adjust your dose safely without knowing exactly what you take.

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Tell us if you take

None of these means you cannot take levothyroxine. Some medicines and levothyroxine affect each other, so knowing what you take helps us look after you more closely. Usually we simply check your blood 6 weeks after you start or stop the other medicine.³

• Estrogen, the birth control pill, or tamoxifen. These raise the protein that carries thyroid hormone in your blood, so you may need more.^2,4
• Testosterone. It lowers that carrier protein, so you may need less.^2,4
• Growth hormone. It changes how your body turns T4 into T3, so your dose may need adjusting.^3,4
• Steroid tablets in high doses. They can lower TSH for a while, so we recheck after you finish them before changing your dose.²
• Acid-reducing medicines (such as omeprazole). They can lower how much you absorb. Take levothyroxine at least 4 hours before them.⁴
• Cholesterol binders (such as cholestyramine) or orlistat (a weight-loss medicine). They lower absorption, so keep them as far apart as possible.^2,4
• Seizure medicines or rifampicin (a tuberculosis medicine). They make your liver break down levothyroxine faster.²
• Antidepressants. Some, such as sertraline, can lower the effect of levothyroxine; older ones called tricyclics can increase side effects on the heart for both medicines.²
• Diabetes medicines. Levothyroxine may make them work less well, so check your blood sugar more often when you start or change dose.²
• Warfarin or another blood thinner. Levothyroxine can make warfarin stronger, so your blood-clotting test should be checked when you start or change dose.²
• Digoxin (a heart medicine). Its level can fall when your thyroid level rises.²
• Ketamine, or medicines that stimulate the heart, before a procedure. Tell the doctor doing the procedure that you take levothyroxine, because together they can raise blood pressure and heart rate.²
• Any new medicine or supplement you start while taking levothyroxine.

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Contact us if you notice

These can be signs that your thyroid level is higher than it should be. They are very rare in people who follow our plan, because we start low, keep your dose within safe limits, and check your blood regularly. We tell you so that you know what to look out for, as part of good care.^1,2

• A pounding, fast or racing heartbeat (palpitations)
• Shaky hands
• Sweating a lot, or feeling too hot
• Trouble sleeping
• Feeling restless, anxious or irritable
• Diarrhoea
• Losing weight while feeling very hungry

If you notice any of these, stop the tablets and contact us. We will arrange a blood test within 1–2 days. Stopping for a day or two is safe, because the medicine stays in your body for about a week. These symptoms often have other causes, such as too much caffeine, and the blood test tells us whether your level is the reason.

If you become pregnant, do not stop the tablets, and tell us straight away. Your body needs more thyroid hormone from early pregnancy, so we will refer you to a specialist doctor that week.^1,5

Go to a hospital the same day if you have chest pain; a fast or irregular heartbeat with shortness of breath; a rash, or swelling of your face, lips, tongue or throat, or trouble breathing or swallowing; a seizure; or sudden confusion or changes in your mood or thinking. Stop the tablets until a doctor has checked you.²

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How to store it

• Keep the tablets in their blister pack, inside the box, to protect them from light and moisture.^1,2
• Store below 30°C. You do not need to keep it in the fridge.¹
• Do not use it after the expiry date on the pack.¹
• Keep it out of the sight and reach of children.^1,2

Medically reviewed by Dr. Chontirot, MD · Medical licence no. 36583 · Last reviewed: 31 October 2026

References

1. Merck Healthcare KGaA. Levothyroxine sodium 50 and 100 microgram tablets — Package insert (Thailand). Revised 21 July 2023.
2. EMD Serono (Merck KGaA). Levothyroxine sodium tablets — Product Monograph including Patient Medication Information. Canada.
3. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12). doi:10.1089/thy.2014.0028
4. Centanni M, Duntas L, Feldt-Rasmussen U, et al. ETA guidelines for the use of levothyroxine sodium preparations in monotherapy to optimize the treatment of hypothyroidism. Eur Thyroid J. 2025;14:e250123. doi:10.1530/ETJ-25-0123
5. The Endocrine Society of Thailand. Recommendations for Thyroid Disorders 2025 (B.E. 2568) (in Thai). Bangkok: The Endocrine Society of Thailand; 2025.

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