The Value of Serum 25-Hydroxyvitamin D Measurements in Hypoparathyroid and Pseudohypoparathyroid

Patients Treated with Calciferol

JAMES BURNS and COLIN R. PATERSON

Department of Biochemical Medicine, Ninewells Hospital and Medical School, Dundee DD1 9SY, Scotland

in 23 patients with hypoparathyroidism or pseudohypoparathyroid- isrn treated with vitamin D, and in whom the dosage was adjusted downward or upward in response to hypercalcemia or hypocalcemia respectively, assays of serum 25-hydroxyvitamin D (25-OHD) were carried out in addition to the •sual serum calcium assays. In 120 assays there was a significant correlation between serum 25-OHD levels and serum calcium levels (corrected for serum albumin). There was, however, no clear distinction between the 25-OHD levels of patients who were hypocalcemic, normocalcemic or hypercalcemic. The highest serum 25-OHD level found in a hypocalcemic patient was 1193 nmol/L and the lowest serum 25-OHD level found in a hypercalcemic patient was 605 nmol/L. It was not possible to predict subsequent episodes of hypocalcemia or hypercalcemia from the serum 25-OHD levels. The 25-OHD assay was found to be useful 0nly in checking compliance. We conclude that the assay of serum 25-OHD is of no more value than serum calcium alone in the man- agement of compliant patients.

KEY WORDS: hypercalcemia; hypocalcemia, hypoparathy- r0idism; pseudohypoparathyroidism; vitamin D; calcifediel

D aily doses of the order of 0.5 to 2.0 mg of v i t amin D (usually ergocalciferol) are required for the t reat-

ment of hypopara thyroid ism (1, 2). The danger of tox- icity when giving such large amounts of v i t amin D and the need to monitor t r ea tmen t by regular mea- surements of serum calcium are well recognised (3, 4). Regular serum calcium assays are also needed to check that patients are not being undertreated.

Serum levels of 25-hydroxyvitamin D (25-OHD) cor- relate well with v i t amin D intake in such pat ients (5, 6). In a previous study (6), in which serum 25-OHD levels and serum calcium levels were measured in hypoparathyroid pat ients receiving fixed doses of ergo- calciferol, the authors suggested tha t measu remen t of serum 25-OHD would enable impending hypercalcemia and hypocalcemia to be recognised, and poor com- pliance and v i t amin D resistance to be differentiated.

We wished to determine whether the addition of a serum 25-OHD assay to our usual clinical practice, in which the v i tamin D dosage is adjusted in relat ion to the patient 's se rum calcium concentration, would prove to be of value in controlling the t r ea tmen t of hypo- parathyroid and pseudohypoparathyroid pat ients on these pharmacological doses of v i t amin D.

Correspondence: Mr. J. Burns, Department of Biochemical Medicine, Ninewells Hospital, Dundee DD1 9SY, Scotland.

Manuscript received April 4, 1985; revised September 24, 1985; accepted October 15, 1985.

Pat ient s and m e t h o d s

In addition to the routine measurements of serum corrected calcium (serum calcium adjusted for serum albumin), 120 serum 25-OHD assays were performed over approximate ly four years on 23 patients, 21 females and two males aged 21 to 71 years (mean = 54 years), who were seen at Ninewells Hospital, Dundee or Per th Royal Inf i rmary. Seventeen pat ients had surgical hypoparathyroidism, four had idiopathic hypo- parathyroidism and two had pseudohypoparathyroidism.

Serum 25-OHD was measured once in five pat ients and between two and 15 t imes in the others. The long- est period of follow-up with the 25-OHD assay was 49 months (mean = 19.4 months) with 12 months or more of follow-up in 15 patients.

The dosage of v i tamin D was adjusted according to the se rum corrected calcium level so tha t in the 22 pat ients t rea ted in Dundee and Perth, the daily dose ranged between 0 and 2.5 mg. Vi tamin D was prescribed for these pat ients in the form of ~calciferol tablets, strong', BP; each tablet contains 1.25 mg of ergocalciferol. One other pat ient who presented at Ninewells Hospital with hypercalcemia had been t rea ted abroad with up to 5 mg daily. Only one pat ient was receiving a calcium supplement.

The depar tmenta l reference range was 2.20 to 2.60 m m o l / L but pat ients were considered to be optimally controlled if thei r serum corrected calcium levels were within the range 2.00 to 2.40 mmol/L. ~Hypocalcemia' refers to a level less than 2.00 mmol /L and ~hypercal- cemia ' to a level grea ter than 2.60 mmol/L. I f a pa t ient had a serum corrected calcium level persis tent ly less t han 2.00 mmol /L , the daily dosage of calciferol tablets was increased by approximate ly 25 per cent. In a pa t ient with a se rum corrected calcium between 2.40 and 2.60 mmol /L , the dose was reduced by approxi- ma te ly 25 per cent. T rea tmen t was stopped al together in hypercalcemic pat ients unti l the serum corrected calcium level fell to about 2.00 mmol /L a t which point the calciferol tablets were resumed, usual ly at ha l f of the previous daily dosage.

Serum calcium and serum albumin were measured with a Technicon SMAC (Technicon Ins t rumen t Co. Ltd., Basingstoke, Han ts RG21 2YE). Serum calcium was corrected for serum albumin by the formula: corrected calcium (mmol/L) = measured calcium (mmol/L) + 0.023 (40 - a lbumin (g/L)).

CLINICAL BIOCHEMISTRY, VOLUME 19, FEBRUARY 1986 49

BURNS AND PATERSON

3500

3000

2500

Serum

25 -hyd roxy 2000 v i tamin D (nmo l / I )

1500

Q

n

o e ~ Q

500 • ; ' "

no • e°4,

0 i n n ~ I i i i i I i i , 1.6 2.0 2.4 2.8 3.2 3.6

Serum ca lc ium (corrected for a l b u m i n ) ( m m o l / I )

Figure 1 - Serum corrected calcium levels and serum 25-OHD levels in 23 hypoparathyroid and pseudohypopara- thyroid patients treated with calciferol. The vertical dashed lines indicate the optimal range for serum corrected calcium in these patients; below this range the dose of calciferol was increased, and above it treatment was reduced or ceased (see text).

Serum 25-OHD was measured by a method described by Skinner and Wills (7). Briefly, serum samples were extracted with methanol /d ie thyl ether, chromato- graphed on 35 × 5 m m columns of silicic acid in open glass columns, and 25-OHD in the purified extract measured by a competi t ive protein binding assay using the v i t amin D binding protein in normal h u m a n serum.

Results

Figure 1 shows the relat ionship between the se rum 25-OHD levels and the serum corrected calcium levels in the 23 patients. There was a significant correlation between these measu remen t s (r = 0.57, p < 0.001). However, considerable overlap was seen between the se rum 25-OHD concentrations found in hypocalcemic, in normocalcemic and in hypercalcemic patients. The highest se rum 25-OHD level found in hypocalcemia was 1193 nmol /L and the lowest serum 25-OHD found in hypercalcemia was 605 nmol/L. Thir teen se rum corrected calcium levels grea ter than 2.60 m m o l / L were measured and consti tuted six episodes of hyper- calcemia observed in five patients. Twenty serum corrected calcium values less than 2.00 m m o l / L were found in the eight episodes of hypocalcemia seen in five patients.

Figure 2 shows the prescribed doses of calciferol, together wi th the serum corrected calcium levels and 25-OHD levels in the two pat ients with the longest follow-up. I t is clear tha t a serum 25-OHD level associ- ated with a high serum calcium level in one pa t ien t (patient 1, 25-OHD = 738 nmol/L; corrected calcium = 3.04 mmol /L) m a y be found with a low serum cal- cium level in another pa t ient (patient 2, 25-OHD =

Caiciferol dose (rag/d)

Patient 1 3

1

0 i , . . . . . . . . . . i , , - - , . . . . . . i

1 5 0 0

Serum 1000 25-hydroxy- vitamin D (nmol/I) 500

a a a u

Pa t i en t 2

Serum calcium (corrected for albumin)

(mmof/I)

3,0

2.6

2.2

1.8 0 20 40 0 20 40

M o n t h s

Figure 2 - Serum corrected calcium levels and serum 25- OHD levels in relation to the prescribed daily doses of cald. ferol in two hypoparathyroid patients with long term follow up. * - - wrong calciferol preparation (too little) dispensed; ~calciferol tablets, high strength BP' (0.25 mg ergocalciferol) given instead of ~calciferol tablets, strong BP' (1.25 mg ergo. calciferol).

725 nmol/L; corrected calcium = 1.86 mmol/L) . In each pa t ien t the pa t te rns of change in serum 25-OHD levels tended to paral lel those in serum corrected calcium. It was not possible to predict subsequent hypocalcemia or hypercalcemia from serum 25-OHD levels.

When poor compliance was suspected in two patients the 25-OHD levels proved useful; they confirmed that one pa t ien t had indeed begun to take his tablets and tha t the other had not been following her t reatment.

Discuss ion

Results for serum 25-OHD from different laborat0. ries are very often not comparable due to the various methods of analysis in use. The most significant con. t r ibu tory factor to the large inter laboratory variation appears to be the method of sample preparat ion, that is, whether some form of chromatography is used before the actual assay (8, 9). Some form of preparative chromatography is essential in order to avoid serious overes t imat ion of 25-OHD, as measured by competitive protein binding assay, due to other v i tamin D metab. olites such as 24,25-dihydroxyvitamin D, 25,26-dihy. droxyvi tamin D and 25-hydroxyvitamin D-26,23-1ac. tone, and also other unidentified nonspecific substance (10).

The sys tem of prepara t ive chromatography which we used (open short silicic acid columns) appears similar to tha t used by Mason and Posen (11) whose work (6)

50 CLINICAL BIOCHEMISTRY, VOLUME 19, FEBRUARY 198~

SERUM 25-OHD IN HYPOPARATHYROIDISM

prompted us to under take the study described here. Differences in resul ts may not therefore be due to different methodological specificity.

Mason and Posen (6) proposed tha t measu remen t s of serum 25-OHD, par t icular ly serial measurements , would be of value in the m a n a g e m e n t of hypopara thy- roid patients on large doses of v i tamin D by mak ing it possible to predict impending hypercalcemic and hypo- calcemic episodes and to dist inguish v i tamin D resis- tance from poor compliance. They studied pat ients on fixed doses of v i tamin D.

In our pat_ients the Prescribed dose of v i tamin D did not always r emain unchanged throughout follow-up. As is our normal practice, the dose was increased if the patient became hypocalcemic, and reduced or stopped if hypercalcemia developed. In individual pat ients we did not find tha t it was possible to predict hypercalcemia and hypocalcemia from the earl ier se rum 25-OHD levels. The changes in serum 25-OHD levels usual ly paralleled those in se rum corrected calcium levels. We did, however, find the assay to be helpful in checking compliance in two patients.

Our experience has been that , in Careful practice where the dosage of v i t amin D is adjusted in response to regular monitor ing of se rum corrected calcium, the assay of serum 25-OHD is of no help in the manage- ment of the compliant patient. 25-OHD provides no information on the adequacy of t r ea tmen t in addition to that provided by serum corrected calcium determi- nations which are easier, cheaper and faster to perform.

References

1. Avioli LV. The therapeutic approach to hypoparathy- roidism. Am J Med 1974; 57: 34-42.

2. Paterson CR. Hypercalcaemia in alphacalcidol therapy. Postgrad Med J 1981; 57: 431-2.

3. Davies M, Adams PH. The continuing risk of vitamin-D intoxication. Lancet 1978; 2: 621-3.

4. Paterson CR. Vitamin-D poisoning: survey of causes in 21 patients with hypercalcaemia. Lancet 1980; 1: 1164-5.

5. Gertner JM, Domenech M. 25-Hydroxyvitamin D levels in patients treated with high-dosage ergo- and chole- calciferol. J Clin Pathol 1977; 30: 144-50.

6. Mason RS, Posen S. The relevance of 25-hydroxycalciferol measurements in the treatment of hypoparathyroidism. Clin Endocrinol 1979; 10: 265-9.

7. Skinner RK, Wills MR. Serum 25-hydroxyvitamin D assay. Evaluation of chromatographic and non-chroma- tographic procedures. Clin Chim Acta 1977; 80: 543-54.

8. Jongen MJM, Van Ginkel FC, van der Vijgh WJF, Kuiper S, Netelenbos JC, Lips P. An international comparison of vitamin D metabolite measurements. Clin Chem 1984; 30: 399-403.

9. Mayer E, Schmidt-Gayk H. Interlaboratory comparison of 25-hydroxyvitamin D determination. Clin Chem 1984; 30: 1199-204.

10. Bouillon R. Radiochemical assays for vitamin D metabo- lites: technical possibilities and clinical applications. J Steroid Biochem 1983; 19: 921-7.

11. Mason RS, Posen S. Some problems associated with assay of 25-hydroxycalciferol in human serum. Clin Chem 1977; 23: 806-10.

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