Which condition would increase the ionized fraction of calcium?

• A. Acidosis
• B. Alkalosis
• C. Normal pH
• D. Hypothyroidism

Answer: (A)

The ionized calcium fraction is controlled by pH through its interaction with albumin. In acidosis (lower pH), more hydrogen ions compete for albumin’s binding sites and reduce the sites available for calcium. This displaces calcium from albumin and shifts more of it into the free, ionized form. So acidosis increases the ionized Ca2+ level. In contrast, alkalosis raises pH, allowing more calcium to bind to albumin and reducing the ionized fraction, while normal pH keeps the usual balance. Hypothyroidism doesn’t directly alter this binding in the same way.