Levothyroxine
Synthroid | Levoxyl | Euthyrox | Tirosint
Overview
Synthetic form of endogenous thyroid hormone T4. Converted to T3 (active form) in peripheral tissues. Binds to thyroid hormone receptors, regulating gene expression for metabolism, growth, and develop
Indications
- Hypothyroidism
- Thyroid-stimulating hormone (TSH) suppression in thyroid cancer
- Myxedema coma (IV)
- Diagnostic use in thyroid suppression tests
Contraindications
- Uncorrected adrenal insufficiency
- Acute myocardial infarction
- Thyrotoxicosis
Classification
Mechanism of Action
Synthetic form of endogenous thyroid hormone T4. Converted to T3 (active form) in peripheral tissues. Binds to thyroid hormone receptors, regulating gene expression for metabolism, growth, and development.
Pharmacodynamics
Increases basal metabolic rate, oxygen consumption, and heat production. Affects protein, carbohydrate, and lipid metabolism.
Pharmacokinetics
- Absorption
- Incomplete and variable (40-80%); affected by food, calcium, iron, antacids. Take on empty stomach.
- Distribution
- Highly protein bound (>99%) to TBG, TBPA, and albumin.
- Metabolism
- Deiodination in liver, kidneys, and other tissues to T3 (active) and reverse T3 (inactive).
- Excretion
- Renal and fecal elimination of metabolites.
- Half-life
- 6-7 days
- Bioavailability
- 40-80%
- Protein Binding
- >99%
Dosage
Typical dosage: 25-200 mcg once daily (individualized by TSH levels)
Available Forms
- Tablet
- Capsule (gel)
- Oral solution
- IV injection
Side Effects
Common
- Usually well-tolerated at correct dose
- Hair loss (transient, first few months)
Serious
- Cardiac arrhythmias (overdose)
- Osteoporosis (chronic excessive doses)
- Angina pectoris
Rare
- Seizures
- Pseudotumor cerebri (pediatric)
Drug Interactions
Calcium reduces levothyroxine absorption. Separate administration by at least 4 hours.
Levothyroxine increases warfarin effect by increasing catabolism of vitamin K-dependent clotting factors. Monitor INR.
Warnings
Thyroid hormones should not be used for the treatment of obesity or weight loss. Doses within the range of daily hormonal requirements are ineffective for weight reduction.
Pregnancy
Category A
Toxicity
Not established (wide therapeutic index when monitored)
Overdose
Symptoms: thyrotoxicosis - tachycardia, tremor, weight loss, diarrhea, fever. Treatment: reduce dose or temporarily discontinue; beta-blockers for sympathetic symptoms.
References
Looking for patient-friendly information? Visit RemedyDoor for easy-to-read guides about this medication.