Levothyroxine Use in Infants and Children with Congenital or Acquired Hypothyroidism

Contributing Editor: Marcia L. Buck, Pharm.D.; Editorial Board: Kristi N. Hofer, Pharm.D.; Michelle W. McCarthy, Pharm.D.


Pediatr Pharm. 2008;14(10) 

In This Article


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]


Comments on Medscape are moderated and should be professional in tone and on topic. You must declare any conflicts of interest related to your comments and responses. Please see our Commenting Guide for further information. We reserve the right to remove posts at our sole discretion.
Post as: