Quite frankly, this is all about billing issues. Your doctor can't bill for two visits on the same day so by splitting up the visits onto separate days, he is able to charge more. Also, two visits means two co-pays if you have a co-pay.
The same goes for the thyroid check. If you just come for a blood test, he bills a much lower fee than if you come for an office visit and blood work.
In the practice where I used to work, patients could not get blood work and an office visit on the same day. Why? Because that way they had to pay a co-pay each time instead of only once.