Ans: C. ThyroxineRef Ganong 25/e p300, 24Ie p300; Harper 30Ie p501).Thyroxine is a lipophilic hormone that acts on nuclear receptor.Steroids and thyroid hormones are distinguished by their predominantly intracellular sites of action, since they can diffuse freely through the cell membrane.They bind to a family of largely cytoplasmic proteins known as nuclear receptors.Upon ligand binding, the receptor-ligand complex translocates to the nucleus where it either homodimerizes, or associates with a distinct liganded nuclear receptor to form a heterodimer.In either case, the dieter binds to DNA to either increase or decrease gene transcription in the target tissue.