What is the primary medication for treating hypothyroidism?

Treatment of Hypothyroidism

Levothyroxine sodium (synthetic T4) is the primary medication for treating hypothyroidism and should be used as first-line therapy for all patients with hypothyroidism. 1, 2

Mechanism and Indications

• Levothyroxine sodium tablets are indicated as replacement therapy in primary (thyroidal), secondary (pituitary), and tertiary (hypothalamic) congenital or acquired hypothyroidism 2
• Hypothyroidism is most commonly caused by chronic autoimmune (Hashimoto's) thyroiditis in iodine-sufficient areas, while worldwide iodine deficiency remains the most common cause 3
• Untreated hypothyroidism can lead to serious complications including heart failure, myxedema coma, and in pregnant women, increased risk of preeclampsia and congenital cretinism 1, 3

Dosing Guidelines

Initial Dosing

• For most adults with primary hypothyroidism, the full calculated starting dose is 1.6 mcg/kg/day 2, 4
• Lower starting doses (25-50 mcg) should be used for:
  • Elderly patients (>70 years) 1, 5
  • Patients with cardiac disease (coronary artery disease, atrial fibrillation) 5
  • Patients with long-standing severe hypothyroidism 5

Administration

• Administer levothyroxine as a single daily dose on an empty stomach, 30-60 minutes before breakfast with a full glass of water 2
• Take at least 4 hours before or after medications known to interfere with levothyroxine absorption 2
• For pediatric patients who cannot swallow tablets, crush the tablet and suspend in 5-10 mL of water for immediate administration 2

Monitoring and Dose Adjustment

• Monitor TSH levels 6-8 weeks after initiating therapy or changing dose 3
• For primary hypothyroidism, target TSH should be within the reference range (0.5-2.0 mIU/L is optimal) 5
• For central hypothyroidism, monitor free T4 levels, which should be maintained in the upper half of the normal range 5
• Once stabilized, annual TSH monitoring is recommended 3
• If TSH remains elevated despite adequate replacement dose, check for:
  • Poor compliance
  • Malabsorption
  • Drug interactions 5

Special Populations

Pregnancy

• Women with hypothyroidism who are pregnant or planning pregnancy should be treated to normalize TSH 1
• Pregnant women with subclinical hypothyroidism should also receive treatment to reduce the risk of pregnancy complications and impaired cognitive development of the offspring 5
• TSH should be monitored every 6-8 weeks during pregnancy as requirements often increase 1

Subclinical Hypothyroidism

• Treatment is recommended for:
  • All patients with TSH >10 mIU/L 5
  • Pregnant women or women planning pregnancy with any TSH elevation 1, 5
  • Consider treatment in symptomatic patients with TSH between 4.5-10 mIU/L 1
• Treatment may be avoided in the very elderly (>85 years) with mild TSH elevations 5

Common Pitfalls and Considerations

• Overtreatment is common and can lead to increased risk of atrial fibrillation and osteoporosis 5
• Inadequate treatment of hypothyroidism is associated with low birth weight in neonates and increased cardiovascular risk 1, 3
• While combination therapy with levothyroxine plus liothyronine (T3) has been studied, levothyroxine monotherapy remains the standard of care due to insufficient evidence supporting superior outcomes with combination therapy 6
• Levothyroxine should not be used for weight loss in euthyroid patients as it is ineffective and potentially dangerous 2
• The annual number of levothyroxine prescriptions has increased significantly, suggesting possible overtreatment of mild or subclinical cases 1

By following these evidence-based guidelines for levothyroxine therapy, clinicians can effectively manage hypothyroidism while minimizing risks associated with under or overtreatment.