Diagnosis: In the setting of long-standing lithium therapy for bipolar disorder, these findings are most consistent with lithium-induced nephropathy.

Teaching Points:

• In patients with chronic lithium use, the presence of numerous microcysts in otherwise normal-sized kidneys is characteristic
  and may obviate the need for renal biopsy.
  
  

• Lithium salts cause direct tubular injury, leading to tubular atrophy and chronic interstitial nephritis. The microcysts formation reflects progressive distal tubular dilatation.