While ACV for thyroid is good, if you are taking medications for diabetes, hypertension, or on diuretics, you may want to avoid apple cider vinegar. ACV lowers blood sugar. So for diabetics, ACV mixed with hypoglycemic medication may cause a significant drop in blood sugar, and make you feel dizzy.
Most hypertensive medicine usually comes with diuretics, and since diuretics make your body release excess water and electrolytes like potassium, ACV, and hypertensive medication/diuretics together may worsen the problem and decrease potassium levels in the body. Therefore, it's always beneficial to check with your doctor before consuming any supplements.