Levothyroxine
Brand names: Synthroid, Levoxyl, Unithroid, Tirosint, Tirosint-SOL. Also called T4, L-thyroxine, thyroxine, LT4.
Thyroid hormone (synthetic T4) 3 graded human studies Last checked
The short answer
From 11 sourcesLevothyroxine is a synthetic copy of thyroxine (T4), the main hormone the thyroid makes; the body converts it to the active T3. It is the standard treatment for hypothyroidism of any cause and, at higher doses, suppresses TSH after surgery for thyroid cancer. It is taken once daily on an empty stomach, and the dose is set by blood tests.
Key facts
Which levothyroxine products are approved, and for what?
Levothyroxine tablets, capsules and oral solution (Synthroid, Levoxyl, Unithroid, Tirosint and generics) are FDA-approved prescription drugs for hypothyroidism and TSH suppression in thyroid cancer. Every label carries a boxed warning that thyroid hormone must not be used to treat obesity or for weight loss.
Status by form
2 forms| Form | Status | Approved for | Boxed warning |
|---|---|---|---|
| TabletsSynthroid, Levoxyl, Unithroid, generics | FDA-approved prescription drug123 | Hypothyroidism (primary, secondary, tertiary; congenital or acquired) in adults and children including newborns; TSH suppression with surgery and radioiodine in well-differentiated thyroid cancer | Yes: not for treatment of obesity or for weight loss; larger doses can cause serious or life-threatening toxicity |
| Capsules and oral solutionTirosint, Tirosint-SOL | FDA-approved prescription drugs45 | Hypothyroidism and TSH suppression in thyroid cancer | Yes: the same weight-loss warning |
What is levothyroxine and how does it work?
The thyroid makes mostly T4, which tissues convert to the active hormone T3. When the thyroid, pituitary or hypothalamus fails, levothyroxine replaces the missing T4. In primary hypothyroidism the pituitary hormone TSH is the usual guide to the dose, alongside free T4 and how the person feels, with checks every few weeks after a change.
How it works
Thyroid hormone (synthetic T4)Levothyroxine is identical to the T4 the thyroid makes. About 80% of the active hormone T3 comes from T4 converted in tissues; T3 and T4 enter the cell nucleus and bind thyroid receptors on DNA, switching on the genes that set metabolic activity. Absorption from the gut ranges from 40% to 80% and rises when fasting.1
What doses are on the label and in studies?
For full replacement in adults the label starts at 1.6 mcg per kg per day and adjusts by 12.5 to 25 mcg every 4 to 6 weeks; older adults and people with heart disease start lower. It is taken once daily on an empty stomach, half an hour to an hour before breakfast.
Label and study amounts, not a dose for you. A prescriber decides whether and how much; these show what the labels print and what trials used.
Doses by use
5 rows| Use | Form | Amount and schedule | Basis |
|---|---|---|---|
| Hypothyroidism, adults (full replacement) | Tablets | 1.6 mcg/kg/day; adjust by 12.5 to 25 mcg every 4 to 6 weeks until euthyroid; doses over 200 mcg/day are seldom required1 | Label |
| Hypothyroidism, older adults or heart disease | Tablets | Start below 1.6 mcg/kg/day; titrate every 6 to 8 weeks1 | Label |
| Pre-existing hypothyroidism in pregnancy, TSH above range | Tablets | Increase by 12.5 to 25 mcg/day; TSH every 4 weeks until stable; back to the pre-pregnancy dose after delivery1 | Label |
| New hypothyroidism in pregnancy | Tablets | 1.6 mcg/kg/day if TSH is 10 mIU/L or more; 1.0 mcg/kg/day if below 101 | Label |
| Subclinical hypothyroidism, adults 65 and older (TRUST trial) | Tablets | 50 mcg daily (25 mcg if under 50 kg or with coronary disease), adjusted to TSH7 | Study |
What is levothyroxine combined with?
Levothyroxine alone is the standard of care. Adding liothyronine (T3) did not improve symptoms, mood or quality of life in a meta-analysis of 11 trials, and a 2021 consensus called for new trials. Oral estrogen raises the binding protein TBG, which can raise the levothyroxine dose a woman needs.
Levothyroxine plus liothyronine (T3)
Some people feel unwell despite normal tests on levothyroxine alone.
A meta-analysis of 11 randomized trials (1,216 patients) found no difference from levothyroxine alone in pain, depression, anxiety, fatigue, quality of life, weight or lipids. The American Thyroid Association keeps levothyroxine alone as the standard, and a 2021 ATA/BTA/ETA consensus agreed there is equipoise for new trials.869
Levothyroxine with oral estrogen
Oral estrogen raises thyroxine-binding globulin (TBG).
The label lists oral estrogens among drugs that raise TBG, which binds more thyroid hormone and can raise the levothyroxine dose needed; TSH is checked after starting or stopping them.1
Checked: TSH after estrogen is started, changed or stopped.
How does levothyroxine relate to fertility and pregnancy?
Untreated hypothyroidism carries risks for mother and baby, and levothyroxine needs usually rise in pregnancy. The label advises checking TSH and free T4 as soon as pregnancy is confirmed and at least each trimester, raising the dose by 12.5 to 25 mcg a day if needed, and returning to the pre-pregnancy dose after delivery.
Fertility, pregnancy and breastfeeding
- Pregnancy
- Dose needs often rise; the label checks TSH and free T4 as soon as pregnancy is confirmed and at least each trimester, and does not stop levothyroxine during pregnancy.1
- Safety in pregnancy
- Clinical experience has not shown more birth defects or miscarriages with levothyroxine; untreated hypothyroidism carries risks for mother and baby.1
- Breastfeeding
- Levothyroxine passes into milk without reported harm to the infant.1
- Fertility
- Too high a dose can cause menstrual irregularities and impaired fertility, as signs of overtreatment.1
What gets checked during treatment?
TSH and free T4 guide the dose: the label titrates every 4 to 6 weeks, or every 6 to 8 weeks with heart disease, and the full effect can take 4 to 6 weeks. Biotin supplements can distort thyroid tests, so the label asks for them to be stopped at least two days before a blood draw.
Monitoring
5 checks| Check | When | Why |
|---|---|---|
| TSH and free T4 | 4 to 6 weeks after each dose change (6 to 8 with heart disease), then periodically1 | To reach and keep a normal TSH; peak effect takes 4 to 6 weeks |
| TSH in pregnancy | As soon as pregnancy is confirmed, every 4 weeks while adjusting, at least each trimester, and 4 to 8 weeks after delivery1 | Needs rise in pregnancy and fall back after delivery |
| Biotin | Stopped at least 2 days before thyroid tests1 | It distorts TSH, T4 and T3 immunoassays |
| Blood sugar (diabetes) | When levothyroxine is started, changed or stopped1 | It can worsen glycemic control |
| Coagulation tests (on warfarin) | When the levothyroxine dose rises or hypothyroidism is corrected1 | It increases warfarin's effect |
What does the research show?
Levothyroxine reliably replaces thyroid hormone, and the American Thyroid Association keeps it as the standard of care. In older adults with only mildly raised TSH, the TRUST trial found no improvement in symptoms or tiredness. In people over 70, current use was linked to more fractures, rising with dose.
Every graded row
3 rows from 3 studiesBGrade BSymptoms with added T3 vs levothyroxine aloneCombination therapyNo effectMeta-analysis · 11 studies
n 1,216 · 2006
No difference in pain, depression, anxiety, fatigue, quality of life, weight or lipids; adverse events similar.
- Design
- Meta-analysis, 11 studies, n = 1,216, 2006
- Population
- Adults with overt hypothyroidism in 11 randomized trials
- Form
- T4 plus T3 vs T4 alone
- Dose
- Various
- Duration
- Various
- Source
- Reference 8 · PubMed 16670166 (opens PubMed)
BGrade BSymptoms and tiredness in older adults with subclinical hypothyroidismSubclinical hypothyroidismNo effectRCT
n 737 · 2017
TSH fell to 3.63 vs 5.48 mIU/L on placebo, but hypothyroid symptom and tiredness scores did not differ.
- Design
- RCT, n = 737, 2017
- Population
- Adults 65 and older with TSH 4.60 to 19.99 mIU/L and normal free T4
- Form
- Tablets
- Dose
- Median 50 mcg daily
- Duration
- 1 year
- Source
- Reference 7 · PubMed 28402245 (opens PubMed)
CGrade CFractures with current use in adults over 70BoneWorsenedObservational
n 213,511 · 2011
Current use vs remote use: fracture odds ratio 1.88; high vs low dose 3.45, medium vs low 2.62.
- Design
- Observational, n = 213,511, 2011
- Population
- Adults 70 and older prescribed levothyroxine, Ontario
- Form
- Tablets
- Dose
- Low, medium and high cumulative doses
- Duration
- Mean 3.8 years
- Source
- Reference 10 · PubMed 21527461 (opens PubMed)
What are the side effects and risks?
Side effects come mostly from too high a dose: palpitations, fast or irregular heartbeat, tremor, anxiety, insomnia, heat intolerance, weight loss and bone loss. Older adults and people with heart disease start low because of atrial fibrillation risk. It can worsen diabetes control. Uncorrected adrenal insufficiency is the one contraindication.
Regulatory status. Levothyroxine products are FDA-approved prescription drugs with a boxed warning against use for obesity or weight loss.14
Most common side effects on the label
Warnings on the label
- Narrow therapeutic index
- Every label opened here says levothyroxine has a narrow therapeutic index: over- or undertreatment can affect growth, heart function, bone, reproductive and cognitive function, and glucose and lipid metabolism, so the dose is titrated carefully and the response monitored.124
- Overtreatment
- Doses beyond replacement cause hyperthyroidism, including arrhythmias, angina and heart failure. In people with normal thyroid function, replacement-range doses do not cause weight loss, and larger doses can be seriously toxic.1
- Heart disease and older age
- Starting below full replacement is advised because of the risk of atrial fibrillation and other cardiac reactions.1
- Bone loss and fractures
- Overtreatment lowers bone density; in adults over 70, current use was linked to more fractures with a dose-response pattern.110
- Adrenal crisis
- With untreated adrenal insufficiency, levothyroxine can trigger acute adrenal crisis; glucocorticoids are replaced first.1
- Myxedema coma
- Oral thyroid hormone is not used to treat myxedema coma.1
Who should not use it (label contraindications)
Uncorrected adrenal insufficiency.1
What does levothyroxine interact with?
Calcium, iron, phosphate binders, bile acid resins, antacids and proton pump inhibitors can reduce absorption, so the label separates levothyroxine from binders by at least 4 hours. It strengthens warfarin, can raise diabetes medicine needs, and oral estrogen raises its binding protein. Levothyroxine is on the Interaction map.
Interactions
6 agents| With | What happens | Evidence |
|---|---|---|
| Binders and mineralsCalcium carbonate, ferrous sulfate, sevelamer, lanthanum, bile acid resins | Reduce absorption; taken at least 4 hours apart from levothyroxine.1 | Drug label |
| Acid reducersProton pump inhibitors, sucralfate, antacids | Lower stomach acid can reduce absorption; TSH is monitored.1 | Drug label |
| Oral anticoagulantsWarfarin | Levothyroxine increases the anticoagulant effect; coagulation tests are watched.1 | Drug label |
| Diabetes medicinesInsulin, oral agents | Glycemic control can worsen and needs can rise.1 | Drug label |
| Oral estrogensEstradiol tablets, combined pills | Raise TBG, which can increase the levothyroxine dose needed.1 | Drug label |
| BiotinHair, skin and nail supplements | Distorts thyroid blood tests; stopped at least 2 days before testing.1 | Drug label |
Check your own medicines and supplements in the Interaction map: every edge is sourced, and nothing you enter leaves your browser.
How do the options compare?
Levothyroxine and liothyronine
| Levothyroxine (T4) | Liothyronine (T3) | |
|---|---|---|
| What it is | The thyroid's main hormone, converted to T3 in tissues | The active hormone itself |
| Usual adult replacement | About 1.6 mcg/kg/day | 25 to 75 mcg daily |
| Dose steps | 12.5 to 25 mcg every 4 to 6 weeks | 25 mcg every 1 to 2 weeks |
| Role | Standard of care for hypothyroidism | Alternative or add-on; also the thyroid suppression test |
Which tools and services help with this?
Frequently asked questions
Common questions
6 answersWhy take levothyroxine on an empty stomach?
Food, calcium, iron and acid reducers can lower how much is absorbed. The label advises taking it once daily, half an hour to an hour before breakfast, and at least 4 hours apart from binders such as calcium or iron.1
Can levothyroxine help with weight loss?
No. Every label carries a warning that thyroid hormone is ineffective for weight loss at replacement doses in people with normal thyroid function, and that larger doses can be seriously toxic.1
How long does levothyroxine take to work?
The label says the peak effect may take 4 to 6 weeks, which is why doses are adjusted every 4 to 6 weeks based on TSH.1
Does mildly high TSH in older adults need treatment?
In TRUST, 737 adults aged 65 or older with subclinical hypothyroidism had no improvement in symptoms or tiredness on levothyroxine after a year compared with placebo.7
Is adding T3 better than levothyroxine alone?
Why stop biotin before a thyroid test?
Biotin interferes with the immunoassays for TSH, T4 and T3 and can give false results; the label asks for it to be stopped at least two days before testing.1
Sources
Sources cited on this page
11 sources- 1Synthroid (levothyroxine sodium) tablets: prescribing information (DailyMed, version effective 2024-02-20)Label AbbVie, 2024
- 2Levoxyl (levothyroxine sodium) tablets: prescribing information (DailyMed, version effective 2020-12-15)Label Pfizer, 2020
- 3Unithroid (levothyroxine sodium) tablets: prescribing information (DailyMed, version effective 2019-06-19)Label Amneal, 2019
- 4Tirosint (levothyroxine sodium) capsules: prescribing information (DailyMed, version effective 2026-03-25)Label IBSA Pharma, 2026
- 5Tirosint-SOL (levothyroxine sodium) oral solution: prescribing information (DailyMed, version effective 2026-03-27)Label IBSA Pharma, 2026
- 6Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacementGuideline Thyroid, 2014 · PMID 25266247
- 7Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism (TRUST)RCT N Engl J Med, 2017 · PMID 28402245
- 8Thyroxine-triiodothyronine combination therapy versus thyroxine monotherapy for clinical hypothyroidism: meta-analysis of randomized controlled trialsMeta-analysis J Clin Endocrinol Metab, 2006 · PMID 16670166
- 9Evidence-Based Use of Levothyroxine/Liothyronine Combinations in Treating Hypothyroidism: A Consensus DocumentGuideline Thyroid, 2021 · PMID 33276704
- 10Levothyroxine dose and risk of fractures in older adults: nested case-control studyObservational BMJ, 2011 · PMID 21527461
- 11Cytomel (liothyronine sodium) tablets: prescribing information (DailyMed, version effective 2026-05-18)Label Pfizer, 2026
What changed
Change log
1 entry| Date | Change | Who and why |
|---|---|---|
| Page created: label status for tablets, capsules and solution, weight-based and pregnancy dosing, combinations with liothyronine and oral estrogen, monitoring, three graded rows (TRUST, combination meta-analysis, fractures), safety and absorption interactions. | Writer: Kymata Health EditorialTrigger: Owner request (2026-10-22): hormone library |
Kymata Health hormone library. Educational information only; it is not medical advice and does not replace a clinician. Doses are shown as labels and studies print them; a prescriber decides whether and how any hormone is used.