Ya, they don't follow the protocol. They should've tested TSH immediately, along with Renal, Lipid profile and even a Cardiac stress test if the patient has history of BP / Corononary artery / Heart disease in his family. Methimazole does great if there is a Thyroid malfunction, resulting in the BP increase. But the BP increase can be due to various reasons, including Diet, Genetics, Heart, Kidney or even Aneurysms. So some immediate tests can quickly point to the most probable cause and more tests can confirm it.
By the way, there is no scientific evidence by NIMH or anyone else that Thiazide cause Diabetes. I've not come across any. But yes, it does temporarily increase the Glucose availability to various channels for transportation. A better way to manage that, is to check for A1C before you put a patient on this. Higher A1C will tell you the person is already pre-diabetic (genetic disposition more than likely if age is around or below 40). So if someone is pre-diabetic, giving them a Cup of Coffee with Sugar in it, or a Plate of Rice Pudding every day will result in full onset of Diabetes within a few years. In which case, you should avoid giving Thiazides. But for the remainder of the 90-95% population, these have been proven excellent first line of defense meds. Thanks.