Table 2.

Unanswered questions in phosphate homeostasis

Questions	Commentary
Is there a phosphate sensor that regulates serum phosphate level?	If so, how does it accommodate changes in age, gender, time of year, time of day?
What drives the circadian rhythm of serum phosphate?	How much of the cycling is diet driven? How much is a reflection of circadian rhythms in hormones? Can it be disrupted, and if so, what are the consequences?
How does serum phosphate relate to bone metabolism?	Serum phosphate is highest in infancy and lowest in old age, reflecting bone metabolism. Is serum phosphate, in fact, simply a reflection of bone metabolism?
How do the mechanisms for total body phosphate homeostasis and the mechanisms regulating the intracellular–extracellular balance of phosphate interact with each other?	To what extent do homeostatic processes influence serum phosphate? Why does serum phosphate at times not reflect total body phosphate stores?
To what extent does serum phosphate reflect dietary phosphate?	Can we prescribe appropriate parameters for dietary phosphate?
How are the functions of the major hormones that regulate phosphate homeostasis integrated?	What are the signals between organ systems and is there a ‘master’ regulator?

This table presents a list of some of the more important areas of uncertainty in our understanding of phosphate homeostasis and regulation of serum phosphate concentration.