Levothyroxine Use in Infants and Children with Congenital or Acquired Hypothyroidism

Pharmacokinetics

Levothyroxine has an oral bioavailability ranging from 40 to 80%. It is absorbed in the intestine, primarily in the jejunum and upper ileum. The presence of food in the gastrointestinal tract decreases absorption. Soybean flour in infant formula, cotton seed meal, walnuts, and dietary fiber may bind levothyroxine and significantly reduce the amount of absorption. Once absorbed, levothyroxine is highly (99.96%) bound to serum proteins, including thyroxine-binding globulin (TBG), thyroxine-binding pre-albumin, and albumin.^[2,3]

Levothyroxine is slowly eliminated, primarily by sequential deiodination to T₃ and reverse T₃. The liver serves as the major site for deiodination, but the process also occurs in the kidneys and other tissues. Thyroid hormones are excreted by the kidneys, but may also undergo conjugation with glucuronides and sulfates or be excreted into the bile and gastrointestinal tract to undergo enterohepatic recirculation. The average elimination half-life of levothyroxine is 6 to 7 days. Because of its long half-life, the full therapeutic effect of a dosing change may not be seen for 4 to 6 weeks.^[2,3]

Pediatr Pharm. 2008;14(10) © 2008  Children's Medical Center, University of Virginia