Levothyroxine

What levothyroxine sodium is and how it works

Levothyroxine is a prescription-only synthetic version of thyroxine (T4), the hormone the thyroid gland normally makes. It is used to replace thyroid hormone in people whose thyroid does not make enough (hypothyroidism), and in some people after thyroid cancer surgery to help suppress TSH. (1)

Take levothyroxine exactly as your prescriber directs, usually once daily on an empty stomach. Your exact dose is individually worked out and adjusted using blood tests (TSH) -- it is not a fixed dose for everyone, and it is never used simply to lose weight.

Mechanism of action: how levothyroxine sodium works

Levothyroxine is synthetic T4, converted peripherally (about 80%) to the more biologically active T3, acting via nuclear thyroid hormone receptors to regulate gene transcription and metabolic rate. (1)

What levothyroxine sodium is used for

Before taking levothyroxine sodium

Do not take / not appropriate for

• Levothyroxine should not be used for weight loss or to treat obesity in people with normal thyroid function -- see the boxed warning above. (all countries) (1)
• Do not start levothyroxine if you have untreated adrenal insufficiency; this needs to be treated with corticosteroid replacement first, under your prescriber's direction, because starting thyroid hormone alone can trigger an adrenal crisis. (all countries) (1) (2)
• Levothyroxine is not started during an acute heart attack, acute inflammation of the heart muscle (myocarditis), or acute inflammation of all layers of the heart (pancarditis); your prescriber will decide when it is safe to start. (United Kingdom) (2)

Tell your clinician about

• Any heart problems, including angina, coronary artery disease, high blood pressure, or arrhythmia.
• Any adrenal gland or pituitary gland problems.
• Whether you are pregnant, planning a pregnancy, or breastfeeding.
• Any osteoporosis or bone density concerns, especially after menopause.
• All other medicines and supplements you take, especially calcium, iron, antacids, proton pump inhibitors, cholestyramine, warfarin, insulin or diabetes medicines, digoxin, and any weight-loss medicines.
• If you take biotin supplements, since these can interfere with thyroid blood tests.

How to take levothyroxine sodium

• Take once daily, preferably on an empty stomach, about 30-60 minutes before breakfast, as your prescriber directs.
• Swallow the tablet with water; try to take it at a consistent time each day.
• Separate levothyroxine from calcium supplements, iron supplements, antacids, and other interacting medicines by at least 4 hours.
• Do not switch between different brands of levothyroxine without your prescriber's involvement, since small differences in absorption between brands may require dose adjustment and follow-up monitoring.
• It can take 4-6 weeks to feel the full effect of a dose, so your prescriber will not usually judge or change your dose sooner than that.

Procedures and treatment changes

Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (3) (2) (4) (5) (6)

Missed dose

If you miss a dose, take it as soon as you remember that day; if you do not remember until the next day, skip the missed dose and continue your normal schedule -- do not take a double dose. Contact your pharmacist or prescriber if you are unsure or miss doses often.

Overdose

Seek urgent medical attention or contact a poison control service if too much levothyroxine has been taken, even without symptoms at first -- symptoms of overdose (such as a fast or irregular heartbeat, chest pain, or severe agitation) can take hours to days to appear. (1) (2)

Side effects

Side effects and their urgency vary; report severe, persistent, or concerning symptoms.

Serious warnings

Interactions

• Calcium supplements, iron supplements, antacids, and cholestyramine reduce absorption of levothyroxine -- take these at least 4 hours apart. (1) (2)
• Proton pump inhibitors (acid-reducing medicines) can reduce levothyroxine absorption; your prescriber may check your TSH more closely if you take one. (2)
• Warfarin's blood-thinning effect can increase; your prescriber may monitor your INR more closely. (1) (2)
• Insulin and other diabetes medicines may need dose adjustment, with closer blood sugar monitoring. (1) (2)
• Digoxin's effect can be reduced when levothyroxine is started or increased. (1)
• High-dose beta-blockers, tricyclic antidepressants, and sympathomimetic medicines can interact with levothyroxine in ways that affect the heart or hormone conversion; tell your prescriber about all medicines you take. (1)
• Biotin supplements can interfere with thyroid blood test results; stop biotin at least 2 days before testing, as your prescriber or lab advises. (1)

Pregnancy, breastfeeding, kidney/liver function, age

Monitoring

Before starting

• Baseline TSH (and free T4 where relevant)
• Assessment for adrenal insufficiency and heart disease
• Review of other medicines, especially those affecting absorption

During treatment

• TSH checked about 6-8 weeks after starting or changing dose in adults, then every 6-12 months once stable.
• In children: TSH and free T4 at 2 and 4 weeks after starting, 2 weeks after each dose change, then every 3-12 months until growth is complete, alongside growth and bone development checks.
• In secondary/tertiary hypothyroidism, free T4 is used to guide dosing rather than TSH.
• Watching for signs of over-treatment (palpitations, tremor, insomnia) or under-treatment (fatigue, weight gain, constipation).

Storage

Store at 20-25°C (68-77°F), with brief excursions to 15-30°C permitted; protect from light and moisture.

Frequently asked questions

References

1. SYNTHROID (levothyroxine sodium) tablet -- full prescribing informationU.S. Food and Drug Administration / DailyMed (NIH), full prescribing information, Synthroid (AbbVie) · United States · dated 2024-02-20 · accessed 2026-07-29 · source 1
2. Levothyroxine 100 micrograms tablets -- Summary of Product Characteristicselectronic medicines compendium (emc), United Kingdom · United Kingdom · accessed 2026-07-29 · source 2
3. Levothyroxine -- Periodic Safety Update Report Single Assessment (PSUSA) and list of nationally authorised productsEuropean Medicines Agency (EMA) · European Union · dated 2016-10-19 · accessed 2026-07-29 · source 3
4. SYNTHROID (levothyroxine sodium) Product MonographHealth Canada product monograph, Synthroid (AbbVie) · Canada · dated 2024-10-08 · accessed 2026-07-29 · source 4
5. Eutroxsig and Oroxine (levothyroxine sodium) -- ARTG registrationsTherapeutic Goods Administration (TGA), Australia · Australia · accessed 2026-07-29 · source 5
6. Levothyroxine, Eltroxin, and Synthroid -- New Zealand Data SheetsMedsafe (New Zealand regulator) · New Zealand · accessed 2026-07-29 · source 6

Brand names by country

Single-ingredient levothyroxine sodium names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.

Priority countries