This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Localization of the receptor of a hormone Xis found to be in the nucleus. What is likely to be X? 
Select and explain. 
A. Adrenaline, B. Insulin, C. Thyroxine, D. FSH
C: Thyroxine
Answer: C: Thyroxine 
Explanation: 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.