I have recently read a paper about how stress affects morning salivary cortisol levels in an individual. I am conducting a study on monozygotic twins and was wondering if cortisol levels would fluctuate similarly in a pair of twins, or whether cortisol and other hormones are unrelated to the genetic makeup of the twins.
Bartels et al. 2003 is a review that reports an estimate of the heritability of the cortisol level of 0.62. In comparison to most traits, it is a relatively high heritability.
So, in short, the answer is "no". Cortisol level variation is lower among twins than among unrelated individuals. If the definition of heritability is a little bit unclear to you, you should have a look at this post.