Drug reference

Levothyroxine

Thyroid Hormone · Thyroid

Also known as Levothyroxine sodium, L-Thyroxine, T4

START

1.6 µg/kg/day PO (typical 50-100 µg) on empty stomach 30-60 min before food

TYPICAL MAX

individualized — titrate to TSH (no fixed daily ceiling)

STOP IF

Untreated adrenal insufficiency · uncorrected thyrotoxicosis

WATCH

TSH at 6-8 weeks · symptoms · cardiac (esp. elderly / CAD)

CDSCO approvedSchedule H

Dose ladderµg/d

Renal dose adjustmenteGFR mL/min/1.73m²

KDIGO 2024 + manufacturer label

Pharmacokineticsplasma · t hours

ONSET: 1w · clinical effect at 1 week
PEAK: 6h · Cmax (T4 serum)
t½: 1w · T4 t½ ~7 days
DURATION: 1w · once-daily steady-state at 4-6 weeks

EXCRETION

Hepatic deiodination → T3 · biliary metabolites

route + CYP

INTERACTIONS

2 major

SEVERE in our sources

PREGNANCY

Category A — dose typically increases 25-30% in pregnancy

FDA category + note

Top interactionssee all 12 →

Calcium CarbonateSevereDatabaseKimi deep-research + Cla
CholestyramineSevereDatabaseKimi deep-research + Cla

Available in India

8 branded formulations. Look up specific brands in the Drugs workspace.

Mechanism

Levothyroxine is a synthetic form of thyroxine (T4), a hormone naturally produced by the thyroid gland. It is converted to its active metabolite, liothyronine (T3), in peripheral tissues. T3 enters the cell nucleus and binds to thyroid hormone receptors, initiating gene transcription and protein synthesis, thereby regulating metabolism, growth, and development.

Indications

Hypothyroidism (all etiologies, except transient hypothyroidism during recovery from subacute thyroiditis)Thyroid-stimulating hormone (TSH) suppression in differentiated thyroid cancer (e.g., papillary, follicular)Congenital hypothyroidismMyxedema coma (IV formulation)Goiter (off-label)hypothyroidismTSH suppression in thyroid cancermyxedema comaCretinismAdult hypothyroidism (Myxoedema)Myxoedema comaNontoxic goiterThyroid nodule (certain benign functioning nodules)Papillary carcinoma of thyroid

Dosing

Adult: Initial: 1.6 mcg/kg/day orally once daily. Adjust in 12.5-25 mcg increments every 4-6 weeks based on TSH levels. Typical maintenance: 100-125 mcg/day. For elderly or patients with cardiovascular disease: start with 12.5-25 mcg/day and titrate slowly.
Pediatric: 0-3 months: 10-15 mcg/kg/day; 3-6 months: 8-10 mcg/kg/day; 6-12 months: 6-8 mcg/kg/day; 1-5 years: 5-6 mcg/kg/day; 6-12 years: 4-5 mcg/kg/day; >12 years: 2-3 mcg/kg/day (or 1.6 mcg/kg/day). Doses are oral, once daily, and titrated based on TSH and clinical response.
Renal adjustment: No specific dose adjustment is generally required for renal impairment; monitor thyroid function as usual. Patients with nephrotic syndrome may require higher doses due to increased protein loss and reduced protein binding of thyroid hormones. Consult endocrinologist for severe cases or complications like myxedema coma with renal failure.…
Geriatric: Start with 12.5–50 μg/day.

Pharmacokinetics

Onset

3-5 days

Peak effect

1-3 weeks (for steady-state blood levels)

Half-life

~7 days

Bioavailability

Incomplete (~80% of the tablet dose is absorbed); slightly increased when taken on an empty stomach.

Protein binding

>99% (primarily to thyroxine-binding globulin, transthyretin, and albumin)

Metabolism

Metabolic inactivation occurs by deiodination and glucuronide/sulfate conjugation in the liver (primary site).

Contraindications

Untreated subclinical or overt thyrotoxicosis
Acute myocardial infarction
Untreated adrenal cortical insufficiency
Hypersensitivity to levothyroxine or any components of the formulation

Side effects

Common

HeadacheNervousnessInsomniaSweatingWeight lossDiarrheaAbdominal crampsTremorsPalpitationsTachycardiaMenstrual irregularitiesHeat intolerance

Serious

Angina pectoris
Myocardial infarction
Congestive heart failure
Arrhythmias
Pseudotumor cerebri
Seizures
Allergic reactions (rare)
Osteoporosis (with long-term suppressive doses)
atrial fibrillation (with overtreatment, especially in the elderly)
osteoporosis (with overtreatment, especially in postmenopausal women)

Pregnancy & lactation

Pregnancy

Category A — dose typically increases 25-30% in pregnancy

Drug interactions

Calcium Carbonate

Severe

Database

Reduced levothyroxine absorption → hypothyroidism

Separate by ≥4 h; monitor TSH

Source: Kimi deep-research + Cla

Cholestyramine

Severe

Database

Resin binds thyroxine in gut

Separate by ≥4 h; monitor TSH

Source: Kimi deep-research + Cla

Aluminum Containing Antacids

Moderate

Textbook

Decreased levothyroxine efficacy, potentially leading to increased TSH levels.