I've been asked to analyze data from an experiment measuring blood hormone levels from rats after a surgery. Hormone measurements were taken 10 days after surgery and 20 days after surgery. To make it more complicated, some of the rats in the surgery group were also subjected to a modified diet. So, there are three groups of rats (sham surgery + regular diet, surgery + regular diet, surgery + modified diet), and hormone measurements were taken at two time points.
What is the correct way to analyze the difference in hormone levels between these groups of rats, both within each time point and between time points? I'm having trouble figuring out the correct ANOVA and/or other method to use. I'm using R.