Serum phosphorus levels (Ps), dietary intake of phosphorus, and renal phosphate 
handling indexes were evaluated in 158 patients with chronic obstructive 
pulmonary disease (COPD) of varying degrees of severity; moreover, skeletal 
muscle phosphorus content (Pm) was measured in muscle samples obtained by 
quadriceps femoris needle biopsy in 14 of the same patients. Hypophosphatemia 
(Ps less than or equal to 2.5 mg/dl) was found in 34 (21.5 percent) of 158 
patients without differences between groups of COPD patients presenting 
increasing severity of respiratory illness. No relationship was found between 
serum levels and dietary intake of phosphorus; hypophosphatemia was associated 
with low renal phosphate threshold (TmPO4/GFR) values in 31 (91 percent) of 34 
patients. The prevalence of hypophosphatemia was significantly higher among COPD 
patients taking one or more drugs commonly used in COPD and known as negatively 
influencing renal phosphate handling: xanthine derivatives, corticosteroids, 
loop diuretics, and beta 2-adrenergic bronchodilators. Short-term administration 
of therapeutic doses of these drugs in COPD patients previously not taking any 
drug reduced TmPO4/GFR values; phosphaturic effect of short-term theophylline 
administration on renal phosphate handling was additive to that of long-term 
assumption of the drug. Muscle phosphorus content was both reduced in COPD 
patients as compared with control subjects and significantly correlated to serum 
phosphorus levels and to TmPO4/GFR values. The present investigation revealed a 
high prevalence of hypophosphatemia among COPD patients as well as a defect in 
renal phosphate reabsorption secondary, at least in part, to pharmacologic 
therapy. Moreover, it also suggests that in COPD patients muscle phosphorus 
content is likely to be reduced in presence of hypophosphatemia.