A 76-year-old man is reviewed in the community mental health team. He has a 5-year history of bipolar affective disorder managed on Lithium 400 mg at night. His latest Lithium level is 0.7 mmol/L (therapeutic range 0.4-1.0). He has CKD stage 3a (eGFR 52) and hypothyroidism (on Levothyroxine). According to NICE and BNF, what monitoring is ESSENTIAL for this patient on long-term Lithium?