Levothyroxine: What it does and how to take it

Levothyroxine is a synthetic thyroid hormone used to replace low thyroid hormone (hypothyroidism). It helps control energy, weight, body temperature, and mood. Many people feel much better once the dose is right, but the drug must be taken consistently to work well.

How to take levothyroxine

Take levothyroxine on an empty stomach, usually 30–60 minutes before breakfast. If mornings don’t work, take it at bedtime at least 3 hours after your last meal. Stick to one routine — taking it the same way every day helps steady blood levels.

Use the exact dose and brand your doctor prescribes. Different brands or formulations can absorb differently, so switching without talking to your prescriber may mean your lab numbers change and your dose needs adjusting.

Common interactions and side effects

Some foods and supplements reduce levothyroxine absorption. Wait 4 hours after taking levothyroxine before using calcium, iron, antacids with aluminum or magnesium, or certain multivitamins. High-fiber meals, soy, and soy-based formulas can also interfere.

Certain medicines change how levothyroxine works. Examples include some cholesterol drugs (like bile acid sequestrants), proton pump inhibitors, estrogen therapy, and iron supplements. Levothyroxine can increase the effect of blood thinners such as warfarin, so your INR may need closer monitoring.

Too much levothyroxine can cause fast heartbeat, sweating, tremors, anxiety, weight loss, or trouble sleeping. Too little leaves you tired, cold, and prone to weight gain. If you notice these signs, tell your doctor — you may need a blood test and dose change.

Monitoring is straightforward: doctors check TSH (and sometimes free T4) about 6–8 weeks after a dose change, then every few months until stable. After stability, testing is usually less frequent unless symptoms change or you have conditions that alter dose needs.

Pregnancy often increases the need for thyroid hormone. If you’re planning pregnancy or already pregnant, tell your provider — many pregnant people need a higher dose and closer monitoring to keep TSH in the target range.

If you miss a dose and it’s less than 12 hours, take it as soon as you remember. If more than 12 hours, skip it and resume your normal schedule; don’t double up. Store tablets at room temperature away from moisture and heat.

Questions? Ask your doctor or pharmacist before changing brands, starting supplements, or adding new medicines. Small adjustments matter with levothyroxine, and regular testing keeps you feeling your best.