What's the meaning of "thin blood" in this context?

I don’t understand the meaning of this expression that I found in a novel I’m reading. A woman who lives in a hot country is speaking with a girl living in England. It’s summer, the girl is warm, the woman instead is cold. The girl is surprised, so the woman says:

Maybe my blood has been thinned by the tropics

I found this expression only in a medical context. A thinned blood is a blood made thinner by an anticoagulant, but I don’t think this is the meaning of the expression in this dialogue. Is it an idiom? Or a common way to say that her blood is accustomed to hot weather?