Digoxin can accumulate to toxic levels in patients with:
Digoxin is primarily excreted unchanged by the kidney, mainly by glomerular filtration. Rate of excretion is altered parallel to creatinine clearance. Its  t½  is prolonged in elderly patients and in those with renal insufficiency: dose has to be reduced. 
Digoxin is a cumulative drug. When maintenance doses are given from the beginning, steady state levels and full therapeutic effect are attained after 4 x t½, i.e. 6-7 days.
Reference: Essentials of Medical Pharmacology Eighth Edition KD  TRIPATHI  page no 560
Renal insufficiency
Chronic hepatitis
Advanced cirrhosis
Chronic pancreatitis