H03: Thyroid therapy

Pharmacometric modeling notes for ATC H03, thyroid therapy.

Modified

September 22, 2026

Also worth knowing. Levothyroxine is treated as a narrow therapeutic index drug, with bioequivalence tightened to 0.90-1.11 [1,2]. Binding globulins shift with pregnancy and oral estrogen, changing unbound fraction [3,4].

References

[1]

Concordet D, Gandia P, Montastruc J-L, Bousquet-Mélou A, Lees P, Ferran A, et al. Levothyrox® new and old formulations: Are they switchable for millions of patients? Clinical Pharmacokinetics 2019;58:827–33. https://doi.org/10.1007/s40262-019-00747-3.

[2]

Concordet D, Gandia P, Montastruc J-L, Bousquet-Mélou A, Lees P, Ferran AA, et al. Why were more than 200 subjects required to demonstrate the bioequivalence of a new formulation of levothyroxine with an old one? Clinical Pharmacokinetics 2019;59:1–5. https://doi.org/10.1007/s40262-019-00812-x.

[3]

Arafah BM. Increased need for thyroxine in women with hypothyroidism during estrogen therapy. New England Journal of Medicine 2001;344:1743–9. https://doi.org/10.1056/NEJM200106073442302.

[4]

Alexander EK et al. Timing and magnitude of increases in levothyroxine requirements during pregnancy in women with hypothyroidism. New England Journal of Medicine 2004;351:241–9. https://doi.org/10.1056/NEJMoa040079.