Thyroxine (levothyroxine) is a synthetic form of the thyroid hormone T4, widely used for treating hypothyroidism. Proper prescribing and monitoring are crucial to achieving optimal therapeutic outcomes and minimizing complications.
1. Indications
a. Primary Hypothyroidism:
- Commonly due to Hashimoto’s thyroiditis, iodine deficiency, or thyroidectomy.
b. Secondary or Tertiary Hypothyroidism:
- Due to pituitary or hypothalamic dysfunction.
c. Suppression Therapy:
- Used in thyroid cancer or non-toxic multinodular goiter to suppress TSH secretion.
2. Dosage
a. Initial Dosing:
- Adults:
- Typical starting dose: 25–50 mcg once daily, adjusted every 4–6 weeks based on TSH and free T4 levels.
- Full replacement dose: 1.6 mcg/kg/day for otherwise healthy individuals.
- Elderly or Cardiac Patients:
- Start with 12.5–25 mcg/day to avoid precipitating angina or arrhythmias.
- Children:
- Dose based on weight and age; typically 10–15 mcg/kg/day in neonates.
b. Maintenance Dose:
- Adults: Typically 100–125 mcg/day for most patients.
c. Timing:
- Take on an empty stomach, at least 30–60 minutes before breakfast or 4 hours after the last meal.
3. Monitoring
a. Initial Monitoring:
- Check TSH and free T4 levels 6–8 weeks after starting therapy or dose adjustment.
b. Long-Term Monitoring:
- Once stable, measure TSH and free T4 levels every 6–12 months or more frequently if symptoms change.
c. Target TSH Levels:
- Normal TSH range: 0.4–4.0 mIU/L.
- Adjust based on clinical context:
- Suppressed TSH for thyroid cancer.
- TSH in the lower-normal range for hypothyroid symptoms.
4. Side Effects
a. Common Side Effects:
- Over-replacement symptoms (due to hyperthyroidism):
- Palpitations.
- Anxiety or irritability.
- Weight loss.
- Insomnia.
b. Rare but Serious Side Effects:
- Cardiac Complications:
- Angina, arrhythmias, or heart failure (especially in elderly or cardiac patients).
- Bone Health:
- Long-term over-suppression may lead to osteoporosis.
5. Contraindications
- Untreated adrenal insufficiency (thyroid hormone increases metabolic demands, which can precipitate adrenal crisis).
- Allergy to levothyroxine or its excipients.
6. Cautions
- Cardiovascular Disease:
- Start at low doses to prevent cardiac strain.
- Pregnancy:
- Safe to use; dose often needs a 25–50% increase due to increased thyroid hormone requirements.
- Malabsorption Syndromes:
- Conditions like celiac disease or Crohn’s disease may require higher doses.
7. Drug Interactions
a. Drugs Reducing Absorption:
- Antacids (calcium carbonate, aluminum hydroxide).
- Proton pump inhibitors (omeprazole).
- Iron supplements.
- Cholestyramine.
- Recommendation: Separate administration by at least 4 hours.
b. Drugs Increasing Clearance:
- Rifampin, phenytoin, carbamazepine.
- Recommendation: Monitor TSH levels closely.
c. Warfarin:
- Thyroxine may enhance the effect of warfarin, increasing bleeding risk.
- Recommendation: Monitor INR when starting or adjusting thyroxine.
8. Special Populations
a. Pregnancy:
- Thyroid hormone is crucial for fetal brain development.
- Monitor TSH every 4–6 weeks and adjust doses to maintain TSH <2.5 mIU/L in the first trimester and <3.0 mIU/L in the second and third trimesters.
b. Elderly:
- Start with a low dose and titrate slowly to prevent cardiac complications.
c. Children:
- Adjust doses based on weight and growth. Monitor TSH and free T4 frequently.
9. Patient Counselling Points
a. How to Take the Medication:
- Take the tablet on an empty stomach with water, at least 30–60 minutes before breakfast.
- Avoid taking it with other medications, coffee, or milk, which can interfere with absorption.
b. Adherence:
- Stress the importance of daily adherence and timing consistency to avoid fluctuations in thyroid levels.
c. Symptoms to Report:
- Signs of over-replacement (palpitations, weight loss, restlessness).
- Signs of under-replacement (fatigue, weight gain, cold intolerance).
d. Long-Term Monitoring:
- Reinforce the need for regular follow-ups to ensure proper dosage and prevent complications.
10. Summary Table
| Parameter | Thyroxine (Levothyroxine) |
|---|---|
| Indications | Hypothyroidism, TSH suppression |
| Initial Dose | 25–50 mcg/day, lower in elderly/cardiac |
| Maintenance Dose | 100–125 mcg/day (adjust by TSH levels) |
| Monitoring | TSH and free T4 every 6–8 weeks initially |
| Side Effects | Hyperthyroid-like symptoms (if over-replaced) |
| Drug Interactions | Antacids, iron, warfarin, rifampin |
Conclusion
Thyroxine is an effective and well-tolerated treatment for hypothyroidism when prescribed and monitored carefully. Individualized dosing, regular monitoring of TSH and free T4, and proper patient counselling are critical to optimizing outcomes and minimizing adverse effects.
