An anatomy and physiology peer of yours makes a claim stating that the hypothalamus is exclusively part of the

Question

CNS. What argument would you make to substantiate that the hypothalamus is an interface between the nervous system and the endocrine system?

Answer 1

The hypothalamus responds to stimuli other parts of the brain. The hypothalamus directly controls the pituitary gland which secretes hormones. This interaction between brain/CNS and hormones (endocrine system) is why you can make that interface argument

Answer 2

The hypothalamus IS exclusively part of the central nervous system.  

Like every other set of neurons in the central nervous system, it is connected to the rest of the body by the inputs it receives from the rest of the body and the outputs it sends to the rest of the body.  These pathways may be multisynaptic.

That is, the hypothalamus is to the endocrine system what the primary motor cortex is to muscles and what the visual cortex is to the retina and its stumuli.  They are all connected to the outside world with varying degrees of complexity.