Navarro-Gonzalez J, Macia M, Mora C, Gallego E, et al
Magnesium and parathyroid hormone levels in peritoneal
dialysis patients
17th Annual CAPD Conference
Perit Dial Int Suppl 1
(Feb) 17:S48 1997
Magnesium is an important constituent of PD fluid but there is no
consensus on the appropriate level. Currently one can obtain fluids
with 0.25, 0.50 or 0.75 mmol/L. The importance of serum magnesium
levels is not known. Normal serum levels are 1.5 - 2.5 mg/dl.
This abstract reports on 13 PD patients' magnesium levels and its
relation to their iPTH levels.
46% of patients (6/13) were
hypermagnesaemic. Patients with iPTH < 120 pg/ml had
significantly higher serum and dialysate Mg levels (2.69 and 1.51
mg/dl) than those with iPTH > 120 pg/ml (2.30 and 1.16 mg/dl).
Looked at another way round, patients with serum Mg < 2.5 mg/dl had
significantly higher iPTH levels than those with serum Mg > 2.5 mg/dl
(276 versus 127 pg/ml). No data are given on individual PD fluid Mg
levels, or dietary intake. The authors conclude that the data suggest
hypermagnesaemia could have a suppressive effect on PTH in PD
patients.
Magnesium is the the fourth most abundant cation in the body, and
within cells is second only to potassium, yet clinicians often ignore
it. A 70 kg human contains 21 to 28 g of magnesium, of which about
60% is in bone, 20% in skeletal muscle, 19% in other cells and only
1% is in extracellular fluid. About 60% of extracellular magnesium
is free (ionised), and it is this portion that is metabolically
active. Dietary magnesium intake varies greatly, especially in
relation to water content. GI absorption is influenced by many
factors including diabetic gastroparesis, calcium, phosphate and
alcohol intake. Interestingly absorption of Mg is not affected by vitamin
D.
Comment: Hypermagnesaemia has been known for over 10 years to
partially
suppress PTH production, but chronic severe magnesium deficiency also
impairs PTH production. Serum free (ionised) magnesium is the best
measure of serum Mg - in this abstract it's not clear which portion
of extracellular Mg was measured, presumably it was the bound
fraction. The association of PTH and Mg does not prove a causal
relationship, although it seems likely.
Nothing really new in this report, but a useful reminder to monitor
Mg if your lab can do it routinely. Unfortunately, serum levels can
be normal even when total body magnesium is low! But...a low serum
level means definite deficiency.
(Alastair J. Hutchinson, M.D., The Royal Infirmary, Manchester,
UK)
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