Graphical Abstract.

Structural formula of the “Janus-faced” THM 3,5-diiodo-L-thyronine (left), which has the same 3,5-iodine substitution pattern as its putative precursors L-T4 and L-T3 at the tyrosyl-ring, but lacks the iodine substitution in 3′-position of the phenolic ring which is essential for binding classical T3 -receptors and conveying canonical and non-canonical thyromimetic effects. The roman god Janus symbolized “duality” and “jani” were ceremonial gateways in ancient Rome typically used for symbolically auspicious entrances or exits. Janus-face (right) source: This image comes from the 4th edition of Meyers Konversationslexikon (1885–90). The copyrights have expired and this image is in the public domain. Wikimedia, Meyers_b9_s0153_b1.png.