Levothyroxine

Levothyroxine

Indication

• replacement therapy for hypothyroidism

Pharmacology

• The principle effect of thyroid hormones is to increase the metabolic rate of body tissues
• Thyroid hormones are also involved in the regulation of cell growth and differentiation
• The incidence of hypothyroidism is about 1 in every 4,000 live births
• The etiology may be primary (eg. agenesis of thyroid glands, defects of hormone synthesis) or secondary (defect in TSH or TRH)

Symptoms of Hypothyroidism

• majority of neonates are asymptomatic
• prolonged neonatal jaundice
• During first 2 weeks of life symptoms of hypothyroidism may include hypotonia, lethargy, feeding difficulties, pallor, hypothermia, and mottling of skin

Diagnosis of Hypothyroidism

1. Routine Screening
   • blood is sent to Ministry of Health for TSH measurement
   • normal TSH < 20 mU/L
   • elevated TSH > 20 mU/L
2. Monitoring of T4, TSH levels

Side Effects

• Side effects result principally from overdosage; signs of thyroid toxicity include tachycardia, palpitations and excessive sweating

Miscellaneous

• The half life of levothyroxine (T4) is longer than 1 week; thus, 4 to 6 weeks are required before steady state is reached. Serum T4 levels determined before steady state is reached would be expected to be subnormal. Therefore, dosage adjustments should be monitored at 3 to 6 week intervals
• Range of normal values:
• TSH:
  • Euthyroid 0.35 - 5.5 mIU/L
  • Suppressed < 0.1 mIU/L
  • Elevated > 15 mIU/L
• T4:
  • 8 - 22 pmol/L
• T3:
  • 3 - 6.5 pmol/L
• Maintain patient at high normal range level of T4 serum level during the first year of life

Dose

• 10 to 14 mcg/kg po, once daily

Supplied As

• 25 mcg/mL oral suspension, prepared by Pharmacy
• A scored 25 mcg tablet is also available