1. Clinical Overview
Levothyroxine Sodium is a synthetic levo-isomer of thyroxine (T4), identical to the endogenous hormone produced by the thyroid gland. It is the standard of care for thyroid hormone replacement therapy in hypothyroidism and thyroid hormone suppression therapy. The 75mcg strength is a commonly prescribed maintenance dose for adults, offering precise titration to achieve euthyroid status.
| Onset | Duration | Bioavailability |
|---|---|---|
| 3-5 days for initial metabolic effects; full therapeutic effect may take 4-6 weeks. | Approximately 7-10 days due to its long half-life. | Approximately 40-80% when taken on an empty stomach; absorption is highly variable and formulation-dependent. |
2. Mechanism of Action
Levothyroxine sodium replaces endogenous thyroxine (T4). It is a prohormone that is converted peripherally to the active hormone triiodothyronine (T3). T3 and T4 bind to nuclear thyroid hormone receptors (TRα and TRβ) and modulate gene transcription, leading to increased metabolic activity in nearly all body tissues.
3. Indications & Uses
- Primary, secondary, and tertiary (hypothalamic) hypothyroidism
- Thyroid hormone suppression therapy in euthyroid patients for management of simple non-toxic goiter, thyroid nodules, or multinodular goiter
- Adjunct to surgery and radioiodine therapy in the management of thyrotropin-dependent well-differentiated thyroid cancer
4. Dosage & Administration
Adult Dosage: Hypothyroidism: Initial dose 1.6 mcg/kg/day (typically 25-50 mcg/day), adjusted in 12.5-25 mcg increments every 4-6 weeks based on TSH. Maintenance dose is highly individualized; 75 mcg is a common maintenance dose. TSH suppression in thyroid cancer: Doses >2 mcg/kg/day to suppress TSH to <0.1 mIU/L.
Administration: Take on an empty stomach, at least 30-60 minutes before breakfast, with a full glass of water. Maintain consistent timing daily. Avoid concomitant intake of food, coffee, calcium carbonate, iron supplements, antacids, proton pump inhibitors, bile acid sequestrants, sucralfate, and soy products for at least 4 hours.
5. Side Effects
Common side effects may include:
- Transient hair loss (especially during initial months of therapy)
- Headache
- Insomnia
- Palpitations
- Increased appetite
- Heat intolerance
- Sweating
- Weight loss
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Calcium Carbonate / Iron Supplements | Decreases levothyroxine absorption by forming insoluble complexes | Major |
| Proton Pump Inhibitors (Omeprazole) | Reduces gastric acidity, may impair dissolution and absorption | Moderate |
| Cholestyramine, Colestipol | Binds levothyroxine in GI tract, reducing absorption | Major |
| Phenytoin, Carbamazepine, Rifampicin | Increase hepatic metabolism of levothyroxine, requiring dose increase | Moderate |
| Estrogens, Oral Contraceptives | Increase thyroxine-binding globulin (TBG), may increase levothyroxine requirement | Moderate |
| Warfarin | Levothyroxine potentiates anticoagulant effect; increased risk of bleeding | Major |
| Digoxin | Levothyroxine may decrease serum digoxin levels | Moderate |
| Antidiabetic Agents (Insulin, Sulfonylureas) | Levothyroxine may worsen glycemic control, requiring dose adjustment | Moderate |
7. Patient Counselling
- DO take the tablet on an empty stomach, first thing in the morning, at least 30-60 min before food/coffee.
- DO take it with a full glass of water.
- DO maintain a consistent daily schedule.
- DO inform all doctors and surgeons about your thyroid medication.
- DO get your TSH checked regularly as advised.
- DON'T stop the medicine even if you feel better; it's a lifelong treatment for hypothyroidism.
- DON'T take it with other medicines/vitamins (especially calcium, iron, antacids). Separate by at least 4 hours.
- DON'T switch brands without consulting your doctor.
8. Toxicology & Storage
Overdose: Symptoms of thyrotoxicosis: tachycardia, palpitations, arrhythmias (AF), angina, tremor, nervousness, insomnia, hyperthermia, heat intolerance, sweating, weight loss, diarrhea, headache, muscle weakness, menstrual irregularities. Severe overdose can lead to thyrotoxic storm (fever, delirium, seizures, coma, cardiovascular collapse).
Storage: Store below 25°C, protected from light and moisture. Keep in the original blister pack or bottle. Keep out of reach of children.