CommentaryA comparison of potency differences among thyroid peroxidase (TPO) inhibitors to induce developmental toxicity and other thyroid gland-linked toxicities in humans and rats

- Three well-known inhibitors of thyroid peroxidase (TPO) were compared.
- Thyroid hormone levels revealed profound differences to variations in potencies.
- Pharmacologic and toxic end points in pregnant humans and rats were compared.
- Propylthiouracil and methimazole caused prenatal toxicity in humans and rats.
- Resorcinol had low potency to inhibit TPO and caused no adverse effects in rats.

The potencies of resorcinol, 6-propylthiouracil (PTU) and methimazole (MMI) for inducing developmental toxicity and neurotoxicity were compared in pregnant rats, regarded as valid model for human thyroid toxicity. Profound differences on maternal thyroid hormone levels (THs), maternal toxicity as well as developmental and neurotoxicity sequelae occurred. Resorcinol affected none of those end points. PTU and MMI caused significant effects. Therapy with either PTU or MMI during the first trimester of human pregnancy can cause reductions of maternal THs, accompanied by disruptions of prenatal development. Clinical MMI studies show sporadic evidence of teratogenic effects, with equivocal relation to thyroid peroxidase (TPO) inhibition. In recent decades no MMI associated prenatal toxicity has been reported, an outcome possibly related to carefully managed therapy. Orally administered resorcinol was rapidly absorbed, metabolized and excreted and was undetectable in the thyroid. In contrast, PTU or MMI accumulated. Resorcinol's potency to inhibit TPO was profoundly lower than that of PTU or MMI. Quantum chemical calculations may explain low resorcinol reactivity with TPO. Thus, distinctions in the target organ and the TPO inhibitory potency between these chemicals are likely contributing to different reductions of maternal THs levels and affecting the potency to cause developmental toxicity and neurotoxicity.