Levothyroxine

Levothyroxine is a laboratory-made (synthetic) version of thyroxine (T4), one of the main hormones your thyroid gland normally makes. When the thyroid can't produce enough of its own, levothyroxine steps in to replace the missing hormone so your body has the amount it needs.

Your body converts some of this T4 into the more active hormone T3, exactly as it would with hormone the thyroid made itself. Because it simply tops up something you're meant to have, a correctly matched dose usually restores normal thyroid levels without a "drug-like" effect — the aim is to help you feel like yourself again, not to add something extra. It is sold as a generic and under brand names such as Synthroid, Levoxyl, Euthyrox, Tirosint, and Eltroxin.

Levothyroxine is the standard, first-choice treatment for hypothyroidism — an underactive thyroid that doesn't make enough hormone. The most common cause in women is Hashimoto's disease, an autoimmune condition in which the immune system gradually lowers the thyroid's output.

By restoring thyroid hormone to a healthy level, it can ease the symptoms of an underactive thyroid such as fatigue, feeling cold, dry skin, constipation, low mood, and unexplained weight gain. Clinicians also prescribe it in other situations decided case by case — for example after thyroid surgery or radioactive-iodine treatment, for some goiters and thyroid nodules, and sometimes during pregnancy when thyroid levels are low. Whether it's right for you, and when to start, is a decision for your clinician based on your symptoms and blood tests.

Levothyroxine may be appropriate for almost anyone whose blood tests and symptoms point to an underactive thyroid, once a clinician has confirmed the diagnosis. For most people it becomes a long-term, often lifelong replacement, because the underlying thyroid problem usually doesn't reverse on its own.

It isn't right for everyone, and some people need a more cautious start or closer monitoring. Tell your doctor before starting if you:

• have a heart condition — including angina, an irregular heartbeat, or a past heart attack — because thyroid hormone increases the heart's workload;
• have an untreated adrenal gland problem or an untreated overactive thyroid, which generally need to be addressed first;
• are older, where clinicians usually begin more gently;
• are pregnant, planning pregnancy, or breastfeeding — levothyroxine is generally considered safe and is often needed in pregnancy, but the dose is watched closely;
• have diabetes or osteoporosis, or take other medicines or supplements (see interactions below).

Importantly, levothyroxine should not be used as a weight-loss aid or an "energy boost" by people whose thyroid works normally — it doesn't help and can be harmful.

Levothyroxine is taken by mouth, usually once a day, as a tablet or (less often) a capsule or liquid. It works as a steady, background replacement, so consistency matters more than anything else. Everything below is general information — always follow your own prescription and your clinician's instructions rather than any numbers, and never change how you take it on your own.

Its absorption is easily thrown off by food, drinks, and other products, so people are usually advised to:

• take it at the same time every day, most often first thing in the morning on an empty stomach, with water, and then wait before eating;
• separate it by several hours from things that block absorption (see the table);
• stick to the same brand or product where possible, since small differences between versions can matter.

Because the right amount is different for every person, your clinician individualizes the dose and adjusts it over time using blood tests — mainly TSH, which shows whether your thyroid levels are too low, too high, or about right. Levels are usually rechecked several weeks after any change, then periodically once you're stable, and more often during pregnancy or if your health changes. It can take a few weeks to feel the full benefit.

When the dose is well matched to your body's needs, levothyroxine usually causes few or no side effects — because it is simply replacing a hormone you should already have. Most "side effects" are actually signs that the dose is too high or too low, which is exactly why monitoring matters. Some people also notice temporary hair shedding or mild symptoms in the first few weeks as their levels settle.

Report symptoms like these to your clinician rather than adjusting the dose yourself — they often just mean a blood test and a small change are due. Over long periods, too much thyroid hormone can also affect bone strength and heart rhythm, another reason doses are kept as accurate as possible.

Levothyroxine is widely used and generally safe when it's monitored, but a few warnings deserve real attention.

• Heart safety. Too much thyroid hormone raises the heart's workload and can trigger chest pain or abnormal heart rhythms, especially in older adults or people with existing heart disease — one reason clinicians often start low and adjust slowly.
• Not a weight-loss or energy drug. Levothyroxine carries a boxed warning — the strongest safety warning U.S. regulators issue — stating it should not be used for obesity or weight loss. In people with normal thyroid function, ordinary doses don't produce healthy weight loss, and larger doses can cause serious, even life-threatening, effects on the heart.
• Allergic reactions. Rarely, people react to a tablet or its inactive ingredients. Seek urgent care for signs such as rash, swelling of the face, lips, or throat, or trouble breathing.
• Long-term over- or under-replacement. Getting too much or too little for a long time carries its own risks — bone and heart effects on one side, a return of hypothyroid symptoms on the other — so keeping up with monitoring is part of the treatment.

Levothyroxine also interacts with many medicines and supplements, and your needs can change in pregnancy or with significant weight change. Beyond the absorption blockers in the table above, tell your doctor and pharmacist about everything you take: it can interact with blood thinners (such as warfarin), some diabetes medicines and insulin, and certain seizure and cholesterol-lowering drugs. Estrogen matters too — menopause hormone therapy and estrogen-containing birth control can raise the amount of thyroid hormone some people need, so mention any change. Biotin supplements are a separate issue: they don't change the medicine itself but can distort thyroid blood-test results, so tell the lab or your clinician if you take them. Never start, stop, or change any of these — or levothyroxine itself — without medical advice, and get urgent help for chest pain, a very fast or irregular heartbeat, or signs of a serious allergic reaction.