Final answer:
The priority laboratory test for a hypertensive patient on a thiazide diuretic is a potassium level test to monitor for hypokalemia, a common side effect that can have serious cardiac complications.
Step-by-step explanation:
The priority laboratory test for a hypertensive patient taking a thiazide diuretic would be a potassium level test. Thiazide diuretics, which include medications like hydrochlorothiazide and chlorthalidone, can lead to an increased excretion of potassium in the urine, potentially causing hypokalemia (low potassium levels in the blood). Monitoring potassium is important since this electrolyte plays a critical role in muscle function, including heart muscles, and its depletion can result in adverse effects such as cardiac arrhythmias and muscle weakness. It is also noteworthy that thiazide diuretics can affect other electrolytes, but potassium monitoring is usually prioritized due to the potential severity of hypokalemia's complications.