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Disclaimer to the On-line Edition
This Manual has been designed for use in the NICU at London Health Sciences Centre (LHSC), London, Ontario, Canada, and represents clinical practice at this institution. The information contained within the Manual may not be applicable to other centres. If users of this Manual are not familiar with a drug, it is recommended that the official monograph be consulted before it is prescribed and administered. Any user of this information is advised that the contributors, Editor and LHSC are not responsible for any errors or omissions, and / or any consequences arising from the use of the information in this Manual.

LEVOTHYROXINE (T4)

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

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

References

  1. McEvoy G K (ed): AHFS Drug Information, American Society of Hospital Pharmacists, 1991.
  2. Roberts, RJ: Drug Therapy in Infants, W.B. Saunders, Toronto, 1984.
  3. Gomella TL (Ed): Neonatology - Management, Procedures, On-Call Problems, Diseases, Drugs, 1992, Appleton and Lange, Norwalk, Connecticut.
  4. Taketomo CK, Hodding JH and Kraus DM: Pediatric Dosage Handbook, Lexi-Comp Inc., Cleveland, 1992.
  5. Young TE and Mangum OB: Neofax - A Manual of Drugs Used in Neonatal Care, Columbus, Ohio: Ross Laboratories, 2010.

Modified: 4 April 2011




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