Levothyroxine sodium

Thyroid hormones

ATC code: H03AA01

Brand names: Synthroid, Levoxyl, Tirosint, Unithroid

Updated 19 Jun 2026 · Methodology

Indications

Post-thyroidectomy state

First line

Lifelong replacement is required after total thyroidectomy. In thyroid cancer, TSH suppression to below 0.1 mU/L is the target in high-risk patients (per AJCC and staging) and 0.1–0.5 mU/L in intermediate risk. After subtotal resection for benign disease, the goal is euthyroidism with the same TSH target as for hypothyroidism.

Sources

ICD-10: E89.0 · ICD-11: 5D40

Primary hypothyroidism

First line

Replacement therapy in primary hypothyroidism – first-line and sole therapy per 2014 and . Starting dose in young adults is 1.6 mcg/kg/day. In elderly and patients with ischemic heart disease, start at 12.5–25 mcg and titrate slowly. TSH target is 0.4–4.0 mU/L, with an upper limit up to 6 mU/L acceptable in the elderly. T4+T3 combinations are not routinely recommended.

Sources

ICD-10: E03 · ICD-11: 5A00.2

Subclinical hypothyroidism

Individual decision

Subclinical hypothyroidism (TSH 4.5–10 mU/L, normal T4) may be treated selectively. recommends therapy when TSH exceeds 10 mU/L, in patients with TPO antibodies and hypothyroid symptoms, and in women who are pregnant or planning pregnancy. Asymptomatic elderly patients are generally not treated because of iatrogenic thyrotoxicosis risk.

Sources

ATA: Guidelines for the Treatment of Hypothyroidism (2014)

ICD-10: E02 · ICD-11: 5A00.22

Marketing claims without evidence base

The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.

Weight loss in euthyroid patients

Not recommended

Using levothyroxine for weight loss in euthyroid patients is not supported by international guidelines and is potentially harmful. Iatrogenic thyrotoxicosis increases the risk of atrial fibrillation, osteoporosis, and cardiomyopathy without producing sustained weight loss.

Sources

ATA: Guidelines for the Treatment of Hypothyroidism (2014)

How to read tiers A–F

AStrong evidence – first or second line in international guidelines
BRecommended in selected cases
CStudy data contradict each other
DBenefit not proven, no documented harm
FDocumented harm, or risk without benefit

Mechanism of action

Synthetic analogue of endogenous thyroxine (T4). Deiodinated in tissues to the biologically active triiodothyronine (T3), which binds nuclear thyroid hormone receptors and regulates genes controlling basal metabolism, growth, and development. Affects carbohydrate, lipid, and protein metabolism, the cardiovascular system, CNS, and skeleton.

Safety

Contraindications

Untreated thyrotoxicosis
Acute myocarditis
Acute myocardial infarction
Untreated adrenal insufficiency
Hypersensitivity

Serious adverse effects

Iatrogenic thyrotoxicosis from chronic overdosing
Atrial fibrillation (on overdose, especially in elderly)
Decreased bone mineral density (on chronic suppressive doses)

Common adverse effects

Symptoms of overdose at excessive doses: tachycardia, tremor, weight loss, sweating, nervousness
Headache

Pregnancy: can you take Levothyroxine sodium?

AEMPS summary of product characteristics (section 4.6): thyroid hormone treatment is not interrupted during pregnancy. No adverse effects on the pregnancy, the fetus or the newborn have been reported with long-term levothyroxine use in pregnancy. Requirements may rise because of oestrogens, so TSH is checked every trimester and the dose is increased if TSH is high; after delivery the pre-pregnancy dose is resumed and TSH is checked 6–8 weeks later. In hyperthyroidism, adding levothyroxine to antithyroid drugs is contraindicated during pregnancy.

Breastfeeding: can you take Levothyroxine sodium?

Hale L1 · Compatible

Compatible with breastfeeding. The amount transferred to milk is physiologically insignificant.

Maternal half-life: 168 h
Maternal peak time: 2.5 h

Calculate RID Safe feeding window Full profile on NIH LactMed ↗

Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.

Practical notes

Timing and administration

Take in the morning on an empty stomach, 30–60 minutes before breakfast, with water only. During this window avoid coffee, tea, juice, milk, soy, and soy products. Alternative: bedtime dosing at least 3 hours after the last meal. Within 4 hours after the dose avoid: iron, calcium, magnesium, omega-3, proton pump inhibitors (pantoprazole, omeprazole), antacids, cholestyramine, and sevelamer — all impair absorption.

Monitoring

Check TSH 6–8 weeks after initiation or dose change. On stable therapy, recheck every 6–12 months. In pregnancy, check TSH every 4 weeks until mid-gestation, then every 6–8 weeks. When switching between brand-name and generic formulations, recheck TSH in 6–8 weeks (bioavailability may differ).

Pregnancy

Levothyroxine requirement increases 25–50 % from early pregnancy. In women with known hypothyroidism, the dose is increased immediately after pregnancy confirmation (2 additional tablets per week or 25–30 % daily dose increase). TSH target follows trimester- and population-specific reference ranges; when unavailable, 2017 sets the upper limit at 4.0 mU/L across all trimesters. The earlier strict target below 2.5 mU/L in the first trimester led to excessive dose escalation and has been revised.

Common myths

Myth: 'hormone medication is dangerous, better use herbs'. Fact: levothyroxine is bioidentical to endogenous thyroxine. Herbal products contain no active hormone and cannot compensate for hypothyroidism.

Myth: 'I can take a break from therapy'. Fact: replacement for primary hypothyroidism is lifelong. Discontinuation leads to symptom return within weeks and risks myxedema coma with prolonged decompensation.

Myth: 'levothyroxine helps with weight loss'. Fact: in euthyroid patients it does not produce sustained weight loss and is harmful (see the 'weight loss in euthyroid patients' indication).

Drug–nutrient interactions

Calcium

Calcium binds levothyroxine in the gut, reducing absorption by 20–25%. Take levothyroxine in the morning on an empty stomach; calcium 4 hours later or in the evening. Check TSH if dosing schedule changes.

Source: ATA: Guidelines for the Treatment of Hypothyroidism (2014)

Iron

Iron binds levothyroxine in the gut. Absorption drops, TSH may rise. Separate by at least 4 hours: levothyroxine in the morning on empty stomach; iron 4 hours later or in the evening.

Source: ATA: Guidelines for the Treatment of Hypothyroidism (2014)

Drug–drug interactions

10 pairs found. Sorted from critical to minor.

MajorCalcium (oral salts: carbonate, citrate, lactate, gluconate)Mineral supplements. Calcium

Mechanism

Calcium carbonate and citrate bind levothyroxine in the gut lumen into a complex that is not absorbed. The AEMPS-approved label names calcium among the substances that reduce absorption of the hormone. The effect depends on timing rather than dose: four hours apart, both are absorbed normally.

Symptoms

Hypothyroid symptoms return on a dose that used to work: fatigue, feeling cold, constipation, weight gain, dry skin, hair loss. TSH climbs at the 6–8 week check.

Management

Space the doses at least 4–5 hours apart, as the label states. In practice: levothyroxine on an empty stomach on waking, calcium with lunch or dinner. If calcium was started recently and TSH has risen, fix the timing first and repeat the test after 6–8 weeks before changing the hormone dose.

Sources

Pair details →Calcium (oral salts: carbonate, citrate, lactate, gluconate) →

MajorIron oral (salts: sulfate, bisglycinate, fumarate, gluconate, polymaltose; heme iron polypeptide, lactoferrin)Oral iron preparations; antianaemic agents

Mechanism

Iron forms a stable, poorly soluble complex with levothyroxine. The AEMPS label lists iron supplements among substances that reduce absorption of the hormone, and NIH ODS names levothyroxine among the drugs iron interacts with. The pairing is common: iron deficiency anaemia and hypothyroidism often occur in the same patient.

Symptoms

TSH rises on an unchanged dose, with fatigue and cold intolerance the patient puts down to the anaemia. The overlap misleads: both conditions cause tiredness.

Management

Levothyroxine in the morning on an empty stomach, iron after a meal or in the evening, 4–5 hours apart. Iron works better with vitamin C and away from dairy. When iron is started in a patient stable on levothyroxine, repeat TSH after 6–8 weeks: the dose may need adjusting for as long as the iron course lasts.

Sources

Pair details →Iron oral (salts: sulfate, bisglycinate, fumarate, gluconate, polymaltose; heme iron polypeptide, lactoferrin) →

MajorOmeprazoleProton pump inhibitors

Mechanism

Levothyroxine needs gastric acidity to dissolve before it can be absorbed. Omeprazole raises stomach pH and the dissolved fraction drops; the AEMPS label names proton pump inhibitors alongside calcium and iron. Unlike chelation, spacing the doses does not solve this one: pH stays high all day.

Symptoms

TSH rises after omeprazole is started and does not return to baseline even when the hormone timing changes. In atrophic gastritis or Helicobacter the drop is larger.

Management

If the inhibitor is staying, check TSH 6–8 weeks after starting it and adjust the levothyroxine dose on the result. When omeprazole stops, repeat the check: absorption recovers and the adjusted dose may now be too high. Also review whether the inhibitor is still needed, since many courses run on without a current indication.

Sources

Pair details →Omeprazole →

MajorWarfarinVitamin K antagonist anticoagulants

Mechanism

Thyroid hormones accelerate catabolism of vitamin K-dependent clotting factors. When levothyroxine starts or its dose increases, warfarin sensitivity rises and INR climbs within 1–4 weeks.

Symptoms

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool. Severe cases include gastrointestinal or intracranial haemorrhage.

Management

When levothyroxine starts or is escalated, check INR at 1, 2, and 4 weeks and titrate warfarin (typically a 10–25% dose reduction). Once thyroid function stabilises, warfarin requirements stabilise too.

Sources

Pair details →Warfarin →

ModerateAshwagandha (Withania somnifera)Dietary supplements, plant adaptogens (local classification)

Mechanism

Ashwagandha stimulates thyroid function: in a trial of 50 people with subclinical hypothyroidism, 300 mg twice daily for eight weeks lowered TSH and raised T3 and T4 versus placebo. On top of an already titrated levothyroxine dose, that push adds up. A case of painless thyroiditis with thyrotoxicosis that resolved after stopping the herb has also been published.

Symptoms

Signs of hormone excess: palpitations, fine hand tremor, insomnia, weight loss, heat intolerance, diarrhoea, irritability. The published case also featured night fever and fatigue.

Management

Anyone on levothyroxine should not add ashwagandha without telling whoever titrates the dose. If it is already in use, check TSH, free T4 and T3 at 6–8 weeks and declare the herb at every test: without that detail the result reads as hormone over-replacement and the dose gets cut for the wrong reason. With palpitations or tremor, stop the ashwagandha first.

Sources

Pair details →Ashwagandha (Withania somnifera) →

ModerateCalcium carbonateMineral supplements. Calcium

Mechanism

Calcium binds levothyroxine in the gut as insoluble complexes, reducing absorption by 20–25%.

Symptoms

Return of hypothyroidism: fatigue, weight gain, cold intolerance, dry skin, low mood. In thyroid cancer, loss of TSH control.

Management

Take levothyroxine in the morning on empty stomach, 30–60 minutes before food and other drugs. Take calcium carbonate 4 hours after levothyroxine (e.g. at lunch or in the evening). Check TSH 6–8 weeks after starting calcium.

Sources

ATA: Guidelines for the Treatment of Hypothyroidism (2014)

Pair details →Calcium carbonate →

ModerateMagnesium (oral salts: citrate, glycinate, oxide, sulfate)Mineral supplements. Macrominerals

Mechanism

Magnesium sits in the same label list as calcium and iron: it reduces levothyroxine absorption by binding in the gut. The effect is somewhat smaller than with calcium, but it counts when magnesium is taken daily at the same hour as the hormone, which is what happens when both live on the bedside table.

Symptoms

Mildly raised TSH across successive checks with no obvious cause and an unchanged hormone dose.

Management

Keep the 4-hour gap. Magnesium is usually taken at night for sleep and night cramps, which fits well with morning levothyroxine. No need to stop it: moving the hour is enough.

Sources

Pair details →Magnesium (oral salts: citrate, glycinate, oxide, sulfate) →

MinorAmoxicillinPenicillins with extended spectrum

Mechanism

Amoxicillin taken concurrently may slightly impair intestinal levothyroxine absorption. With separated dosing, the effect is clinically negligible.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

Take levothyroxine in the morning on an empty stomach, 30–60 minutes before food and other medications. Take amoxicillin 4 hours later or at lunch. For short amoxicillin courses (5–10 days), no special TSH monitoring needed.

Sources

Pair details →Amoxicillin →

MinorAscorbic acidVitamins. Antioxidants

Mechanism

Ascorbic acid raises gastric acidity and improves levothyroxine absorption in patients with atrophic gastritis or hypochlorhydria. In healthy patients the effect is minimal.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

In patients with confirmed atrophic gastritis or PPI intolerance, ascorbic acid 500 mg with levothyroxine improves absorption. In healthy patients, no specific adjustment is needed. Monitor TSH on the usual schedule.

Sources

FDA: Synthroid (levothyroxine sodium) prescribing information (2017)

Pair details →Ascorbic acid →

MinorAtorvastatinHMG-CoA reductase inhibitors (statins)

Mechanism

Hypothyroidism itself raises . Restoring euthyroid status on levothyroxine reduces statin need. No direct pharmacokinetic interaction.

Symptoms

The combination usually causes no specific symptoms. Each drug's individual side effects remain.

Management

On starting levothyroxine in patients on stable atorvastatin, check lipids 6–8 weeks after TSH normalises. If drops below target, atorvastatin can be reduced. In newly diagnosed hypothyroidism with hypercholesterolaemia, delay statin start until euthyroid status is achieved.

Sources

Pair details →Atorvastatin →

Label mentions54 mentions

Verbatim lines from the interaction sections, in the language of the label

From this drug’s label

ModerateAmiodarone
«Amiodarona y medios de contraste yodados: Debido a su elevado contenido en yodo, pueden provocar tanto hipertiroidismo como hipotiroidismo. Se requiere una especial precaución en los casos de bocio…»
AEMPS, section 4.5 · Open on CIMA
Moderateantacidsclass named in the label
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Proton Pump Inhibitors Sucralfate Antacids (e.g., aluminum & magnesium hydroxides, simethicone) Gastric acidity is an essential requirement for…»
FDA, section 7 · Open on DailyMed
Moderateanticoagulantesclass named in the label
«Derivados cumarínicos: Las hormonas tiroideas pueden potenciar la acción de los anticoagulantes administrados de forma concomitante, puesto que levotiroxina desplaza a los anticoagulantes de su unión…»
AEMPS, section 4.5 · Open on CIMA
Moderateanticoagulantsclass named in the label
«Oral Anticoagulants: Levothyroxine sodium increases the response to oral anticoagulant therapy. Therefore, a decrease in the dose of anticoagulant may be warranted with correction of the hypothyroid…»
FDA, section 7 · Open on DailyMed
Moderateantidiabéticosclass named in the label
«Antidiabéticos: Levotiroxina puede reducir la eficacia hipoglicemiante de los antidiabéticos (tales como metformina, glimepirida, glibenclamida así como insulina) . Por tanto, en los pacientes…»
AEMPS, section 4.5 · Open on CIMA
Moderateantidiabeticsclass named in the label
«Antidiabetic Therapy: Addition of levothyroxine sodium therapy in patients with diabetes mellitus may worsen glycemic control and result in increased antidiabetic agent or insulin requirements…»
FDA, section 7 · Open on DailyMed
ModerateCarbamazepine
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Other drugs: Carbamazepine Furosemide (> 80 mg IV) Heparin Hydantoins Non-Steroidal Anti-inflammatory Drugs - Fenamates These drugs may cause…»
FDA, section 7 · Open on DailyMed
Moderatedigitálicosclass named in the label
«Preparados digitálicos: Si se inicia el tratamiento de levotiroxina en pacientes digitalizados, puede ser necesario un ajuste de la dosis de digital. Los pacientes con hipertiroidismo pueden…»
AEMPS, section 4.5 · Open on CIMA
ModerateDigoxin
«Preparados digitálicos: Si se inicia el tratamiento de levotiroxina en pacientes digitalizados, puede ser necesario un ajuste de la dosis de digital. Los pacientes con hipertiroidismo pueden…»
AEMPS, section 4.5 · Open on CIMA
ModerateFurosemide
«Se requiere monitorización en pacientes que reciben la administración concomitante de levotiroxina y medicamentos (como amiodarona, inhibidores de la tirosina quinasa, salicilatos y furosemida en…»
AEMPS, section 4.4 · Open on CIMA
ModerateFurosemide
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Other drugs: Carbamazepine Furosemide (> 80 mg IV) Heparin Hydantoins Non-Steroidal Anti-inflammatory Drugs - Fenamates These drugs may cause…»
FDA, section 7 · Open on DailyMed
ModerateGlibenclamide
«Antidiabéticos: Levotiroxina puede reducir la eficacia hipoglicemiante de los antidiabéticos (tales como metformina, glimepirida, glibenclamida así como insulina) . Por tanto, en los pacientes…»
AEMPS, section 4.5 · Open on CIMA
ModerateGlimepiride
«Antidiabéticos: Levotiroxina puede reducir la eficacia hipoglicemiante de los antidiabéticos (tales como metformina, glimepirida, glibenclamida así como insulina) . Por tanto, en los pacientes…»
AEMPS, section 4.5 · Open on CIMA
Moderateinhibidores de la bomba de protonesclass named in the label
«Inhibidores de la bomba de protones (IBP): Se recomienda un control regular de la función tiroidea y un control clínico durante el tratamiento concomitante. Puede ser necesario aumentar la dosis de…»
AEMPS, section 4.5 · Open on CIMA
Moderateinsulinclass named in the label
«Antidiabetic Therapy: Addition of levothyroxine sodium therapy in patients with diabetes mellitus may worsen glycemic control and result in increased antidiabetic agent or insulin requirements…»
FDA, section 7 · Open on DailyMed
ModerateInsulin
«Antidiabéticos: Levotiroxina puede reducir la eficacia hipoglicemiante de los antidiabéticos (tales como metformina, glimepirida, glibenclamida así como insulina) . Por tanto, en los pacientes…»
AEMPS, section 4.5 · Open on CIMA
Moderateinsulinaclass named in the label
«Antidiabéticos: Levotiroxina puede reducir la eficacia hipoglicemiante de los antidiabéticos (tales como metformina, glimepirida, glibenclamida así como insulina) . Por tanto, en los pacientes…»
AEMPS, section 4.5 · Open on CIMA
ModerateMagnesium hydroxide
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Proton Pump Inhibitors Sucralfate Antacids (e.g., aluminum & magnesium hydroxides, simethicone) Gastric acidity is an essential requirement for…»
FDA, section 7 · Open on DailyMed
ModerateMetformin
«Antidiabéticos: Levotiroxina puede reducir la eficacia hipoglicemiante de los antidiabéticos (tales como metformina, glimepirida, glibenclamida así como insulina) . Por tanto, en los pacientes…»
AEMPS, section 4.5 · Open on CIMA
ModerateOrlistat
«El hipotiroidismo y/o el control reducido del hipotiroidismo pueden ocurrir cuando se administran conjuntamente orlistat y levotiroxina (ver sección 4.5). Se debe aconsejar a los pacientes que toman…»
AEMPS, section 4.4 · Open on CIMA
ModerateOrlistat
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Orlistat Monitor patients treated concomitantly with orlistat and levothyroxine sodium for changes in thyroid function.»
FDA, section 7 · Open on DailyMed
ModeratePhenytoin
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Other drugs: Carbamazepine Furosemide (> 80 mg IV) Heparin Hydantoins Non-Steroidal Anti-inflammatory Drugs - Fenamates These drugs may cause…»
FDA, section 7 · Open on DailyMed
Moderateproton pump inhibitorsclass named in the label
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Proton Pump Inhibitors Sucralfate Antacids (e.g., aluminum & magnesium hydroxides, simethicone) Gastric acidity is an essential requirement for…»
FDA, section 7 · Open on DailyMed
ModerateRitonavir
«Inhibidores de la proteasa: Se ha observado que el efecto de la levotiroxina puede disminuir cuando se administra junto a lopinavir/ritonavir. Por ello, se deberá monitorizar rigurosamente los…»
AEMPS, section 4.5 · Open on CIMA
ModerateSemaglutide
«Semaglutida: La administración concomitante de semaglutida puede afectar la exposición a levotiroxina. La exposición total (AUC) de tiroxina (ajustada por niveles endógenos) aumentó en un 33% después…»
AEMPS, section 4.5 · Open on CIMA
Amiodarone
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Other drugs: Amiodarone Amiodarone inhibits peripheral conversion of levothyroxine (T4) to triiodothyronine (T3) and may cause isolated…»
FDA, section 7 · Open on DailyMed
Amitriptyline
«Antidepresivos tricíclicos: Levotiroxina aumenta la sensibilidad de los receptores de catecolaminas acelerando la respuesta a los antidepresivos tricíclicos como amitriptilina o imipramina.»
AEMPS, section 4.5 · Open on CIMA
Amitriptyline
«Antidepressant Therapy: Concurrent use of tricyclic (e.g., amitriptyline) or tetracyclic (e.g., maprotiline) antidepressants and levothyroxine sodium may increase the therapeutic and toxic effects of…»
FDA, section 7 · Open on DailyMed
antiácidosclass named in the label
«Fármacos que contienen aluminio, fármacos que contienen hierro o cálcio: La absorción de levotiroxina puede verse reducida por la ingesta concomitante de agentes de unión de ácido gástrico que…»
AEMPS, section 4.5 · Open on CIMA
anticonceptivos hormonalesclass named in the label
«Anticonceptivos que contengan estrógenos y medicamentos utilizados en terapia hormonal sustitutiva en menopausia»
AEMPS, section 4.5 · Open on CIMA
antidepresivos tricíclicosclass named in the label
«Antidepresivos tricíclicos: Levotiroxina aumenta la sensibilidad de los receptores de catecolaminas acelerando la respuesta a los antidepresivos tricíclicos como amitriptilina o imipramina.»
AEMPS, section 4.5 · Open on CIMA
barbituratesclass named in the label
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Phenobarbital Rifampin Phenobarbital has been shown to reduce the response to thyroxine. Phenobarbital increases L-thyroxine metabolism by…»
FDA, section 7 · Open on DailyMed
barbitúricosclass named in the label
«Los fármacos inductores de enzimas como rifampicina, carbamazepina, fenitoína, barbitúricos y productos que contienen hierba de San Juan (Hypericum perforatum) pueden aumentar el aclaramiento…»
AEMPS, section 4.5 · Open on CIMA
betabloqueantesclass named in the label
«Propiltiouracilo, glucocorticoides y beta-bloqueantes: Estas sustancias inhiben la transformación periférica de T4 a T3 y puede conducir a una concentración sérica reducida de T3 .»
AEMPS, section 4.5 · Open on CIMA
Biotin
«Drug-Laboratory Test Interactions: Biotin»
FDA, section 7 · Open on DailyMed
Carbamazepine
«Los fármacos inductores de enzimas como rifampicina, carbamazepina, fenitoína, barbitúricos y productos que contienen hierba de San Juan (Hypericum perforatum) pueden aumentar el aclaramiento…»
AEMPS, section 4.5 · Open on CIMA
corticosteroidesclass named in the label
«Propiltiouracilo, glucocorticoides y beta-bloqueantes: Estas sustancias inhiben la transformación periférica de T4 a T3 y puede conducir a una concentración sérica reducida de T3 .»
AEMPS, section 4.5 · Open on CIMA
Dexamethasone
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Glucocorticoids (e.g., Dexamethasone ≥ 4 mg/day) Short-term administration of large doses of glucocorticoids may decrease serum T3…»
FDA, section 7 · Open on DailyMed
digitalisclass named in the label
«Digitalis Glycosides: Levothyroxine sodium may reduce the therapeutic effects of digitalis glycosides. Serum digitalis glycoside levels may decrease when a hypothyroid patient becomes euthyroid…»
FDA, section 7 · Open on DailyMed
fibratesclass named in the label
«Drugs Known to Affect Thyroid Hormone Pharmacokinetics: Clofibrate Estrogen-containing oral contraceptives Estrogens (oral) Heroin / Methadone 5-Fluorouracil Mitotane Tamoxifen These drugs may…»
FDA, section 7 · Open on DailyMed

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Frequently asked

What is Levothyroxine sodium used for?

Levothyroxine sodium is evaluated for the following indications with varying evidence strength: Primary hypothyroidism (evidence tier A), Post-thyroidectomy state (evidence tier A), Subclinical hypothyroidism (evidence tier B). See the full indication matrix with dosing and citations above on this page.

What are the side effects of Levothyroxine sodium?

Common side effects of Levothyroxine sodium (≥ 1 in 100): Symptoms of overdose at excessive doses: tachycardia, tremor, weight loss, sweating, nervousness, Headache. See the Safety section for uncommon and serious reactions.

Is Levothyroxine sodium safe during pregnancy?

Is Levothyroxine sodium compatible with breastfeeding?

Compatible with breastfeeding. The amount transferred to milk is physiologically insignificant.

Who should not take Levothyroxine sodium?

Levothyroxine sodium is contraindicated in: Untreated thyrotoxicosis; Acute myocarditis; Acute myocardial infarction; Untreated adrenal insufficiency; Hypersensitivity. Full list in the Safety section.

hormone medication is dangerous, better use herbs

levothyroxine is bioidentical to endogenous thyroxine. Herbal products contain no active hormone and cannot compensate for hypothyroidism.

I can take a break from therapy

replacement for primary hypothyroidism is lifelong. Discontinuation leads to symptom return within weeks and risks myxedema coma with prolonged decompensation.

levothyroxine helps with weight loss

in euthyroid patients it does not produce sustained weight loss and is harmful (see the 'weight loss in euthyroid patients' indication).

Official sources

Direct links to regulator labels. Open in a new tab.

AEMPS / CIMA
FDA / DailyMedLabel dated 05/19/2026
GRLSLabel dated 04/23/2025

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