Administration of intravenous ferric carboxymaltose (FCM) for iron-deficient 
patients suffering heart failure with reduced ejection fraction (HFrEF) has been 
associated with transient hypophosphatemia. We sought to investigate and model 
the effect of intravenous FCM on phosphate levels in iron-deficient patients 
with HFrEF. In this single-center retrospective study, serum phosphate levels, 
recorded for clinical reasons, were collected out to 60 days following 
intravenous FCM. Hypophosphatemia was defined as a nadir serum phosphate level 
<0.64 mmol/L. This was further categorized as severe (0.4 to <0.64 mmol/L) and 
extreme (<0.4 mmol/L). Factors associated with hypophosphatemia and change in 
serum phosphate over time were explored. Of 173 patients included, 47 (27%) 
experienced hypophosphatemia, 44 (25%) were classified as severe, and 3 (2%) 
extreme. Risk of hypophosphatemia was increased for patients with a creatinine 
clearance between 60 and <90 mL/min (odds ratio, 2.3; 95% confidence interval, 
1.0-5.5), while <60 mL/min was protective. The median time to nadir in patients 
who experienced hypophosphatemia was 8 (interquartile range, 4-16) days, with a 
return to baseline levels at 6 weeks. Biochemically relevant hypophosphatemia is 
common following a single dose of intravenous FCM. The median time to nadir was 
8 days, and creatinine clearance may influence phosphate levels following 
intravenous FCM. These observations support the need to increase awareness among 
clinicians administering intravenous FCM to iron-deficient patients with HFrEF.