system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
All of the following statements regarding calcium are true except:
assistant
The correct answer is  Calcium is the major extracellular divalent cation, which serves as a second messenger for the activity of many hormones. In the plasma, 45% of calcium is ionized, and the one that is available for exerting physiological effects and when this fraction is reduced it produces hypocalcemic symptoms. Regulation of the extra-cellular calcium is under tight endocrine control and its intestinal absorption efficiency is inversely proportional to calcium intake so that a diet low in calcium leads to a compensatory increase in fractional absorption, due to activation of vitamin D. More than 98% of filtered calcium is reabsorbed in the tubules, which is regulated by parathyroid hormone.