Did you get it?Question: What's the difference between a vitamin and a hormone?
Answer: You can’t hear a vitamin.
As for me, I had to make a considerable effort to figure out why the character who'd heard it found it hilarious. It seems to me I got it, yet I'm not dead sure I got it right. So I'd like to check on what is my best guess with those forumers who can get the humor of the joke straight away.
For those who, like me, fail to think of the collocation the joke might be based on, here's the link that may help.
P.S. BTW, the book is Private Pleasures by Lawrence Sanders. I wonder where it appeared first - in the novel (1994) or on Reddit or elsewhere on the net)))