Endogenous deiodinase-mediated conversion of T4 to T3 stimulates calcification in corals

In vertebrates, thyroid hormones (TH) are synthesized predominantly as the prohormone thyroxine (T4), which is converted into biologically active triiodothyronine (T3) by iodothyronine deiodinases (DIOs). Although iodothyronines have been detected across several invertebrate lineages, their origin, activation mechanisms, and physiological roles, particularly in early-diverging metazoans, remain poorly understood. Here, we showed that T4 and T3 stimulated calcification in the octocorals Corallium rubrum and Sarcophyton sp. In vivo injection, competitive ELISA measurements, pharmacological inhibition experiments, and phylogenetic analyses demonstrated that evolutionarily conserved DIO homologues mediated the conversion of T4 into T3. Feeding experiments further revealed that planktonic prey constituted a major exogenous source of iodothyronines and shaped their concentrations in coral tissues. Together, these findings indicate that dietary iodothyronines and their DIO-mediated activation regulate calcification in octocorals, supporting conservation of key elements of a cellular iodothyronine pathway linking environmental availability to coral biomineralization.