1. Clinical Overview
Levothyroxine sodium is a synthetic levo-isomer of the endogenous thyroid hormone thyroxine (T4). It is the standard of care for thyroid hormone replacement therapy in hypothyroidism. The 37.5 mcg strength is a crucial intermediate dose, often used for fine-titration in patients requiring doses between 25 mcg and 50 mcg, or for patients with underlying cardiac conditions requiring a very gradual dose escalation.
| Onset | Duration | Bioavailability |
|---|---|---|
| 3-5 days | Up to 7 days after discontinuation due to long half-life. | Approximately 70-80% when taken on an empty stomach. Absorption is reduced by food, coffee, and certain medications. |
2. Mechanism of Action
Levothyroxine sodium is a prohormone. It is converted peripherally (mainly in liver and kidneys) to the active hormone triiodothyronine (T3) by deiodinase enzymes. T3 then enters the cell nucleus and binds to specific thyroid hormone receptors (TRα and TRβ), which are bound to thyroid hormone response elements (TREs) on DNA. This binding modulates gene transcription, leading to increased synthesis of numerous proteins responsible for the metabolic effects of thyroid hormones.
3. Indications & Uses
- Primary hypothyroidism (autoimmune thyroiditis, post-thyroidectomy, post-radioiodine therapy)
- Secondary (pituitary) or tertiary (hypothalamic) hypothyroidism
- Suppression of TSH in euthyroid patients with a history of well-differentiated thyroid cancer (papillary/follicular) to prevent tumor recurrence
- Myxedema coma (requires intravenous formulation and higher initial doses)
4. Dosage & Administration
Adult Dosage: Starting dose for healthy adults <60 yrs: 1.6 mcg/kg/day (approx 100-125 mcg/day). For new diagnoses, a full replacement dose is often safe. For those with cardiac risk or long-standing severe hypothyroidism, start low (e.g., 25-50 mcg/day). The 37.5 mcg dose is used for titration between 25 mcg and 50 mcg, or as a maintenance dose for small-framed individuals.
Administration: Take on an empty stomach, at least 30-60 minutes before breakfast, with a full glass of water. Maintain a consistent daily schedule. Do not take with coffee, soy products, high-fiber meals, or calcium/iron supplements (separate by at least 4 hours).
5. Side Effects
Common side effects may include:
- Transient hair loss (especially during initial months of therapy)
- Mild weight loss
- Increased appetite
- Palpitations (if dose is excessive)
- Insomnia (if dose is excessive or taken late in the day)
- Heat intolerance
- Sweating
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Calcium Carbonate / Iron Supplements | Markedly decrease levothyroxine absorption by forming insoluble complexes. | Major |
| Proton Pump Inhibitors (Omeprazole, Pantoprazole) | Reduce gastric acidity, potentially impairing dissolution and absorption. | Moderate |
| Cholestyramine, Colestipol | Bind levothyroxine in the gut, reducing absorption. | Major |
| Estrogens / Oral Contraceptives | Increase serum thyroxine-binding globulin (TBG), increasing total T4 requirement. | Moderate |
| Phenytoin, Carbamazepine, Rifampicin | Increase hepatic metabolism of levothyroxine, increasing dose requirement. | Moderate |
| Amiodarone | Inhibits peripheral conversion of T4 to T3, can cause both hypothyroidism or hyperthyroidism. | Major |
| Beta-blockers (e.g., Propranolol) | May inhibit peripheral conversion of T4 to T3. Also used to control symptoms of thyrotoxicosis. | Moderate |
| Antidiabetic Agents (Insulin, Sulfonylureas) | Thyroid hormones may increase blood glucose, necessitating dose adjustment of antidiabetics. | Moderate |
| Warfarin | Levothyroxine potentiates anticoagulant effect by increasing catabolism of vitamin K-dependent clotting factors. Increased risk of bleeding. | Major |
| Digoxin | Hyperthyroidism reduces serum digoxin levels; initiating levothyroxine may lower digoxin efficacy. | Moderate |
| SSRIs (e.g., Sertraline) | May slightly increase levothyroxine requirement. | Minor |
7. Patient Counselling
- DO take the tablet first thing in the morning, on an empty stomach, at least 30-60 min before food, coffee, or other medications.
- DO take it with a full glass of water.
- DO maintain a consistent daily routine.
- DO inform all your doctors and your surgeon/dentist that you are on this medication.
- DO get your TSH checked regularly as advised by your doctor (typically every 6-12 months once stable).
- DON'T stop taking the medicine even if you feel better. It is a lifelong replacement therapy for most.
- DON'T take it with coffee, milk, soy milk, or high-fiber cereals.
- DON'T take calcium or iron supplements within 4 hours of levothyroxine.
8. Toxicology & Storage
Overdose: Symptoms of thyrotoxicosis: tachycardia, palpitations, arrhythmias (atrial fibrillation), angina, heart failure, tremors, nervousness, insomnia, hyperthermia, heat intolerance, sweating, weight loss, diarrhea, headache, menstrual irregularities. In severe cases: psychosis, seizures, coma.
Storage: Store in a cool, dry place, below 25°C, protected from light and moisture. Keep the bottle tightly closed. Keep out of reach of children. Do not use after the expiry date printed on the pack.