Levothyroxine: Drug Information
1. Drug Name
Generic Name
Levothyroxine
Brand Name(s)
- Pakistan: BrandX
- India: BrandY
- US: Synthroid, Levoxyl, Tirosint
- UK: Eltroxin, Euthyrox
2. Pharmacological Class
Class
Thyroid hormone
Subclass
Synthetic thyroid hormone
3. Mechanism of Action
Levothyroxine is a synthetic form of thyroxine (T4), a hormone produced by the thyroid gland. It acts by replacing or providing thyroid hormone that is normally produced by the body to regulate energy and metabolism.
4. Indications
Primary Indications
- Treatment of hypothyroidism
- Suppressive therapy for thyroid cancer
Off-label Uses
- Used in euthyroid patients with nodular thyroid disease to prevent goiter growth
- Adjunctive therapy for depression refractory to standard therapy
5. Dosage and Administration
Standard Dosage
The dosage of levothyroxine is individualized based on the patient's age, weight, and the severity of hypothyroidism.
Route of Administration
Oral administration, usually as tablets or capsules.
Special Populations
- Pediatric: Dosage adjustments may be necessary in children to maintain normal growth and development.
- Geriatric: Start with lower doses due to potential sensitivity.
6. Pharmacokinetics
Absorption
Levothyroxine is well absorbed from the gastrointestinal tract.
Distribution
It is widely distributed throughout the body, including the brain and breast milk.
Metabolism
Metabolized mainly in the liver, kidneys, and other tissues.
Excretion
Primarily excreted in the urine.
Half-life
The elimination half-life is approximately 6 to 7 days in euthyroid patients; it is shorter in patients with hypothyroidism.
7. Contraindications
- Hypersensitivity to levothyroxine or any component of the formulation
- Untreated thyrotoxicosis
8. Warnings and Precautions
- Thyrotoxicosis: Monitor for signs and symptoms of hyperthyroidism.
- Cardiovascular effects: Use with caution in patients with cardiovascular disorders.
- Adrenal insufficiency: Initiate therapy cautiously in patients with adrenal insufficiency.
9. Side Effects
Common Side Effects
- Weight loss
- Palpitations
- Tremors
Serious Side Effects
- Cardiac arrhythmias
- Osteoporosis (with long-term use)
- Exacerbation of diabetes mellitus
10. Drug Interactions
Major Interactions
- Antacids
- Iron supplements
Moderate Interactions
- Warfarin
- Phenobarbital
Minor Interactions
- Estrogen-containing preparations
- Protein-bound drugs
11. Pregnancy and Lactation
Pregnancy Category
Category A - Safe to use during pregnancy.
Breastfeeding Considerations
Levothyroxine is excreted into breast milk but at doses that are unlikely to have a clinically significant effect on the infant, especially when breastfeeding is well-established.
12. Patient Counseling Information
- Take levothyroxine as prescribed; do not exceed the recommended dose.
- Take on an empty stomach, at least 30 minutes to 1 hour before breakfast.
- Notify healthcare provider of any changes in symptoms, especially chest pain, rapid heartbeat, or excessive sweating.