Effects of furosemide administration on ionized calcium in normal subjects.

Dr. Jose Manuel Hermida, Andres Mellado 7, E-28015 Madrid (Spain) Dear Sir, There is evidence that furosemide, when given orally to normal subjects, can induce an increase in serum parathyroid hormone (PTH), even though changes in serum calcium have not been observed [1, 2]. Calcium-lowering effects have been reported in hypercalcemia [3] and in hypoparathyroidism [4]. We have studied the response to furosemide infusion (0.5 mg/kg) in 8 normal subjects who had been instructed to follow a diet of 800–1,000 mg of calcium for 3 days before the study and of 3 g of NaCl the day before the study. Blood samples were taken before and 2 h after infusion. Urine samples were collected before (two 45-min periods) and after (three 60-min periods) furosemide administration. We measured in both blood samples: serum calcium (atomic absorption spectropho-tometry), phosphorous, ionized calcium (Ca++), pH, and Ca++ at pH 7.4 (ICAI radiometer) [5], as well as C-termi-nal PTH (IRE). Urine samples were analyzed for calcium. Laboratory data obtained are shown in table I. In our study we found an initial fall in Ca++ 2 h after administration of furosemide. This decrease in Ca+ + may be caused by the hypercalciuria that furosemide induces [6]. It is known that furosemide tends to produce metabolic alkalosis [7], and, since alkalosis may, in turn, reduce Ca++ [8], a fall in Ca++ could also be justified by this mechanism. We found a significant increase in serum pH, together with a decrease in Ca++, whilst Ca++ at pH 7.4 showed no change. Our data suggest that alkalosis induced by furosemide can play a role in its calciumlowering effect. Table I. Serum and urinary values before and after furosemide infusion