Beth summed it up well. I'll add, the doctor tests for a variety of things on HRT. The upside to taking HRT is knowing how healthy your body is pretty much performing as a whole, since every month they're (competent doctors) are checking for many things. My doctor tests for:
Total Testosterone, Estradiol, Prolactin: for obvious reasons.
Complete Blood Count: A breakdown of how many blood cells (white and red) are in your body, what's in them, how healthy they are, etc. If you have abnormal levels of this, it indicates not only a general unhealthiness, but can possibly mean you are at risk of clots or strokes
Comprehensive Metabolic Panel: A breakdown of different minerals and enzymes in your body that measures how well your liver, kidney, and digestive system are functioning. The breakdown measures such things as chloride, potassium, protein, calcium and sodium in your serum, how much CO2 is in your blood, and other enzymes which are only produced in large amounts if your liver has a problem.
Lipid Panel: Measures your cholesterol and triglycerides which tries to determine if you are at risk for clots.
Like I said, you get a pretty good health screen. Since you are getting it done every 3 months, chances are if anything bad develops towards your health that they'll catch it in the beginning stages. That's the good thing about not self medicating.