General rules:
• Hydrogen ion concentration (H) is directly proportional with potassium ion concentration (K)
• Sodium ion concentration (Na) is inversely proportional with potassium ion concentration (K) and hydrogen ions (H)

Hypokalemia is due to:
• alkalosis (decreased H leads to decreased K)
• diuretics (all exc. K-sparing ones)
• diarrhea
• corticosteroids (they increase Na, thus K is decreased)

Hyperkalemia (ttt)
• Insulin + glucose: to shift K into the cells
• NaHCO3: to reverse the accompanying acidosis
• IV Ca gluconate: to protect the heart from asystole
• furosemide: to get rid of K

Aldosterone-related
• Aldosterone excess » Na retention, K loss
• Aldosterone deficiency » Na loss, K retention (a cause of hyperkalemia)

Misc.
• Thiazides are calcium sparing, so never use them in hypercalcemia
• Hypomagnesemia » hypokalemia & hypocalcemia (main clinical picture is due to these rather than hypomagnesemia)