Levothyroxine

Is levothyroxine safe?

Yes, for nearly everyone it is prescribed for. Levothyroxine is a synthetic copy of thyroxine (T4), the hormone an underactive thyroid stops making. When the dose matches the body's needs, it does not add anything foreign to the system and causes almost no side effects. Most people take it daily for life.

The catch is precision. The label describes it as a narrow therapeutic indexOverdose marginHow far a normal dose is from a harmful one. A wide margin means an accidental extra dose is unlikely to cause serious harm.More in the glossary drug: too little brings back tiredness and weight gain, too much acts like an overactive thyroid and, over years, strains the heart and bones. Interactions, recalls and product switches can push people out of range without them noticing. We score it 73, close to sertraline (72) and below metformin (80), mainly because of the boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary, the recall record and the many absorption interactions.

Getting the dose right

The Synthroid label sets the full adult replacement dose at 1.6 mcg per kg of body weight a day, so about 110 mcg for a 70 kg adult. Older people and anyone with heart disease or at risk of atrial fibrillation start lower and go up more slowly. Doses rise in 12.5 to 25 mcg steps every 4 to 6 weeks, guided by the TSH blood test, because the full effect of a change takes that long to show. More than 200 mcg a day is seldom needed.

Overtreatment is the long-term risk that matters. In a study of 17,684 people on thyroxine in Tayside, Scotland, those whose TSH was fully suppressed had 1.6 times the risk of heart rhythm problems and about twice the fracture risk of people in the normal range. A TSH that was low but not suppressed carried no clear extra risk. Regular blood tests are the safeguard, at least yearly once stable.

What gets in the way: food, supplements and other drugs

Most interactions stop the tablet being absorbed. The label advises:

• Take it on an empty stomach, 30 to 60 minutes before breakfast, at the same time each day.
• Keep it at least 4 hours apart from calcium carbonate, iron tablets, antacids, phosphate binders and cholesterol resins such as cholestyramine.
• Expect possible dose changes with proton pump inhibitors such as omeprazole, sucralfate and orlistat.
• Soy flour, walnuts, high-fibre food and grapefruit juice can reduce or delay absorption.

Other interactions go the other way. Levothyroxine increases the effect of warfarin, so blood clotting tests need watching when the thyroid dose changes. It can worsen blood sugar control in diabetes. Rifampicin and phenobarbital speed up its breakdown, and the MHRA has reported falling thyroxine levels with ritonavir-containing medicines.

The biotin problem

Biotin, sold in high doses for hair, skin and nails, does not change how levothyroxine works. It changes the blood test. Many lab tests for TSH, T4 and T3 use a biotin-based method, so high biotin intake can make results look falsely abnormal. The label says to stop biotin at least 2 days before a thyroid test. Tell whoever orders the test if you take it, including in a multivitamin.

Recalls and switching products

Levothyroxine tablets contain microgram amounts, so small manufacturing errors matter. openFDA lists 81 recall records for levothyroxine between 2022 and 2026, all Class II, mostly for tablets that tested below or above their stated strength. Recent ones include Viatris and Mylan in November 2024, Accord in April and June 2025 and again in August 2026, and Major Pharmaceuticals in July 2026. A Class II recall means temporary or reversible harm is possible but serious harm is unlikely.

In the UK the MHRA said in 2021 that generic prescribing suits most people, but some report symptoms when their pharmacy changes the product. If that happens, ask for a thyroid test and consider staying on one product.

Who should be careful

Talk to your doctor or pharmacist before any change if you have heart disease, are over 65, have diabetes, take warfarin, or are pregnant or planning pregnancy, which often raises the dose needed. Levothyroxine is contraindicatedContraindicationA situation in which a medicine should not be used, because the risk clearly outweighs any benefit, such as a known allergy to it.More in the glossary in uncorrected adrenal insufficiency. Never take it to lose weight: the boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary says large doses can cause life-threatening toxicity. If a child swallows tablets, call Poison Help (1-800-222-1222) in the US or 111 in the UK even if they seem well, because symptoms can appear days later.

How we scored it

Dependence (95) and track record (95) are the strongest factors. Regulatory warnings (45) are the weakest, reflecting the boxed warningBoxed warningThe strongest safety warning the US FDA puts on a medicine's label, printed inside a black border at the top.More in the glossary against weight-loss use and the run of potency recalls. Interactions (55) and overdose marginOverdose marginHow far a normal dose is from a harmful one. A wide margin means an accidental extra dose is unlikely to cause serious harm.More in the glossary (60) reflect how easily absorption problems or dose errors push levels out of range.