Is Thyroxine a hormone?
It is necessary to distinguish between classical and non-classical actions of thyroid hormones.

Classical action involves nuclear THR (or TR) receptors (TR alpha1, TR beta1, TR beta2). They are activated by T3, 3,5-T2, TRIAC and 3,5-DIAC, and blocked by rT3 and rTRIAC. T4 and TETRAC hardly have an effect on nuclear thyroid hormone receptors, but they are converted in the periphery and in target tissues by deiodinases to T3 and 3,5-T2, and TRIAC and 3,5-DIAC, respectively. From the classical perspective, T4 and TETRAC are therefore prohormones and not hormones in the narrower sense.

Non-classical action denotes binding of thyroid hormones to integrin receptors on the cell membrane. The resulting non-genomic actions may be directly mediated by T4 and TETRAC and don't require deiodination. From the non-classical perspective T4 and TETRAC are hormones sensu stricto.


Figure from Hoermann et al. 2015 (CC BY license)

References


Hoermann R, Midgley JE, Larisch R, Dietrich JW. Homeostatic Control of the
Thyroid-Pituitary Axis: Perspectives for Diagnosis and Treatment. Front
Endocrinol (Lausanne). 2015 Nov 20;6:177. doi: 10.3389/fendo.2015.00177.
eCollection 2015. Review. PMID 26635726
Davis PJ, Goglia F, Leonard JL. Nongenomic actions of thyroid hormone. Nat Rev Endocrinol. 2016 Feb;12(2):111-21. doi: 10.1038/nrendo.2015.205. PMID 26668118.