Soy - Levothyroxine Sodium Interaction Details

Interaction Details

Levothyroxine Sodium is classified as belonging to the following category: Levothyroxine (Synthroid, Others)

Soy products might reduce the absorption of levothyroxine in some patients.
Preliminary clinical research and a case report suggest that soy-based formulas inhibit the absorption of levothyroxine in infants with congenital hypothyroidism. A levothyroxine dosage increase may be needed for infants with congenital hypothyroidism while using soy-based formulas, and the dose may need to be reduced when soy-based formulas are no longer administered. However, in postmenopausal adults, clinical research shows that taking a single dose of soy extract containing isoflavones 60 mg along with levothyroxine does not affect the oral bioavailability of levothyroxine.

Interaction Rating

Moderate

Likelihood of Occurrence

Possible

Interaction has been documented in animal or in lab research, or the interaction has been documented in humans but is limited to case reports or conflicting clinical research exists

References

Jabbar MA, Larrea J, Shaw RA. Abnormal thyroid function tests in infants with congenital hypothyroidism: the influence of soy-based formula. J Am Coll Nutr. 1997;16(3):280-2.
Conrad SC, Chiu H, Silverman BL. Soy formula complicates management of congenital hypothyroidism. Arch Dis Child. 2004;89(1):37-40.
Chorazy, P. A., Himelhoch, S., Hopwood, N. J., Greger, N. G., and Postellon, D. C. Persistent hypothyroidism in an infant receiving a soy formula: case report and review of the literature. Pediatrics 1995;96(1 Pt 1):148-150.
Fruzza AG, Demeterco-Berggren C, Jones KL. Unawareness of the effects of soy intake on the management of congenital hypothyroidism. Pediatrics. 2012;130(3):e699-702.
Persiani S, Sala F, Manzotti C, et al. Evaluation of levothyroxine bioavailability after oral administration of a fixed combination of soy isoflavones in post-menopausal female volunteers. Drug Res (Stuttg). 2016;66(3):136-40.

Soy - More Interactions

Soy interacts with 602 drugs

Interaction Rating Key

These severity listings are for informational use only. Never start, stop or otherwise change your therapy before speaking with your provider.

Major	The combined use of these agents is strongly discouraged as serious side effects or other negative outcomes could occur.
Moderate	Use cautiously under the care of a healthcare professional or avoid this combination. A significant interaction or negative outcome could occur.
Minor	Be aware that there is a chance of an interaction. Watch for warning signs of a potential interaction.
Unknown	No interactions have been reported or no interaction data is currently available.