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Levothyroxine

Brand names: Synthroid, Levoxyl, Unithroid, Euthyrox

Levothyroxine is a man-made thyroid hormone that replaces the hormone your thyroid isn't making enough of.

Brand namesSynthroid, Levoxyl, Unithroid, Euthyrox
Type of medicineThyroid hormone
Prescription?Prescription
ImportantThis page is general information, not a prescription. Don't start, stop, or change a medicine without talking to your provider or pharmacist.

Overview

It is the same as the T4 hormone your thyroid makes. Most people with an underactive thyroid take it for life.

Your provider adjusts the dose based on blood tests (TSH), usually every 6 to 8 weeks after a change.

What it's used for

  • Underactive thyroid (hypothyroidism)
  • After thyroid surgery or radioactive iodine treatment
  • Some types of goiter and thyroid cancer

How to take it

  • Take it once a day on an empty stomach, with a full glass of water, usually 30 to 60 minutes before breakfast.
  • Take it the same way every day.
  • Separate it by 4 hours from calcium, iron, antacids, and multivitamins with minerals.
  • Don't switch brands without talking to your provider; small differences matter.

Your dose depends on your health and other medicines. Always follow your prescription label, and ask your pharmacist if anything is unclear.

Side effects

Common side effects

  • Few side effects at the right dose
  • If the dose is too high: shakiness, trouble sleeping, sweating, fast heartbeat, weight loss
Call 911Chest pain, a racing or irregular heartbeat, or fainting
Call your provider todayFeeling very nervous, shaky, hot, or unable to sleep after a dose change

Warnings

  • It should not be used to lose weight. Large amounts can cause serious harm.
  • Tell your provider if you have heart disease; your dose may be started low.
  • Symptoms can take several weeks to improve.

Other medicines, foods, and alcohol

  • Calcium, iron, antacids, soy, and high-fiber foods can block absorption.
  • Blood thinners like warfarin may need monitoring when your dose changes.
  • Some seizure medicines and estrogen can change how much you need.

Tell your provider and pharmacist about every medicine, vitamin, and supplement you take.

Pregnancy and breastfeeding

Levothyroxine is safe in pregnancy and the dose often needs to go up. Tell your provider as soon as you know you're pregnant.

Missed doses, overdose, and storage

If you miss a dose

Take it as soon as you remember, unless it's almost time for your next dose. Then skip the missed dose. Don't take two doses at once.

If someone takes too much

Call Poison Control at 1-800-222-1222 or get help online at poison.org. Call 911 if the person collapses, has a seizure, has trouble breathing, or can't be woken.

Storing it

Store at room temperature, away from heat and moisture (not in the bathroom), and out of reach of children and pets.

Conditions it's used for

Sources

Last updated 2026-11-13.