Genesis of Hyperparathyroidism

ERIC REISS, M.D.

JANET M. CANTERBURY, M.S.

Chicago, lllinois

From the Department of Medicine, Michael Reese Hospital and Medical Center and the University of Chicago Pritzker School of Medicine, Chicago, Illinois 60616. This work was supported by Research Grant AM-8572 from the National Institute of Arthritis and Metabolic Diseases, Public Health Service. Requests for reprints should be addressed to Dr. Eric Reiss, De- partment of Medicine, Michael Reese Hos- pital and Medical Center, Chicago, Illinois 60616.

The radioimmunoassay of PTH has provided a powerful new tool for clinical investigation. Although much remains to be learned about the details of the assay, rapid progress is being made in many laboratories. The subtlety of PTH response in various disease states is becoming evident. The great variety of clinical circumstances under which primary hyperparathy- roidism occurs suggests multiple etiologies of the disease. It is possible that primary hyperparathyroidism is the consequence of long-term adaptation and therefore represents not a primary disease but the consequence of other diseases. If this proves true, the study of primary parathyroid disease will assume a new meaning in clinical practice.

No comprehensive theory exists concerning the genesis of primary hyperparathyroidism. When the disease was first recog- nized it was assumed that a benign neoplasm of one parathyroid gland and the resulting hormone excess satisfactorily ac- counted for the clinical manifestations. As in the case of other forms of neoplasm, speculation about etiology was useless. With increasing experience gained by investigators throughout the world during the past forty years, it has become evident that primary hyperparathyroidism is a highly pleomorphic disease. It occurs in many varied and apparently unrelated clinical circumstances and possesses an astonishing variety of clinical expressions. Dr. Keating, whose memory we honor by this Sym- posium, was impressed by this diversity of clinical settings and expressions. In his last paper he urged that the traditional simplistic view of hyperparathyroidism be abandoned and in- dicated the need for learning more about the natural history of the disease [l].

In this presentation we review some data on the genesis of renal hyperparathyroidism and suggest that primary hyperpara- thyroidism may be a consequence of long-term stimulation of the parathyroid glands more commonly than has been supposed in the past. The data are based on the radioimmunoassay tech- nic of Berson and associates [2], an extraordinarily powerful tool in clinical investigation. Because interpretation of results hinges critically upon the assay, validating data as well as some current difficulties are reviewed.

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THE ASSAY

As performed in our laboratory, the radioimmuno- assay of parathyroid hormone (PTH) possesses an unusually high capacity for discriminating be- tween serums of patients with primary hyper- parathyroidism and serums of patients with hy- poparathyroidism [3] (Figure 1). Increased levels of PTH have been demonstrated in all eighty-four patients with surgically confirmed primary hyper- parathyroidism studied thus far. PTH has been undetectable by this assay in the fifteen serum samples from hypoparathyroid subjects studied. This’ high degree of discrimination, which is con- trary to the experience of others, must be in large part related to the binding characteristics of the antibody used for the assay. In part, how- ever, these clear results may reflect an element of selection. We have excluded serums referred from other centers unless the disease was surgically confirmed. In defjning the group of normal sub- jects, we have eliminated all subjects who may have some form of secondary hyperparathyroidism. This precaution may account for the narrow range of values in normal subjects.

100

I I I I z ; NORMAL

2

,,YPER;;;$;ROIDISM HYPOPARATHYROIDISM

: 1200&__+ . a ‘=doo’

UNhKASURAEL E

Figure 1. Results of radioimmunoassay of PTH in nor- mal subjects’ and patients with primary hyperparathyroid- ism and with hypoparathyroidism.’ The units of PTH in this and subsequent figures relate the potency of test serums to’ that af an arbitrarily selected hyperparathyroid serum [4]. The normal subjects were healthy laboratory and office personnel, house officers and hospitalized patients free of demonstrably abnormalities of calcium and phos- phorus metabolism on careful clinical evaluation. In the hyperparathyroid group, the presence of the disease was surgically confirmed in all instances. Qf the hypoparathy- roid patients, four had the idiopathic form and eleven had the postoperative form. This plot is an extension of data reported previously [3,5].

90

60

70

60

IL

G 50

20

01”“““” IO 30 so 70 90

BPTH ypgIO.3 ml I I I I I

0 IO 20 30 40

TCA-HPTH m/g/0.3 ml Figure 2. Comparison of standard curves obtained with bovine (BPTH) and human (TCA-HPTH) hormones. The value of 100 per cent was assigned to assays containing no unlabeled hormone. The bovine hormone used for labeling and displacement was a highly purified prepara- tion (Wilson Laboratories). The human hormone for dis- placement was b’Riordan’s preparation purified through the TCA precipitation step [8]. It is estimated to be 2.5 per cent pure. To convert the human scale to highly puri-

fied hormone equivalents, the human abscissa scale must be divided by 40. When this is done it is evident that the same depression of the B:F ratio is obtained by ten times the amount of highly purified human hormone as of highly purified bovine hormone.

The results obtained in one patient remain unex- plained [3]. He was a middle-aged ‘man with a his- tory of recurrent nephralithiasis and persistent normocalcemia. Serum PTH was markedly in- creased on many occasions, and no evidence of secondary hyperparathyroidism could be un- covered. At surgery, four normal parathyroid glands were located and inadvertently removed. Subsequently, unequivocal clinical tiypoparathy- roidism developed, yet increased levels of PTH were demonstrated repeatedly by radioimmunoas- say in the postoperative period. The possibility of supernumerary parathyroid glands appears to be excluded by the climnical course, and no evidence of

The American Journal of Medicine

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0 I 2 3 4

TIME-HOURS Figure 3. Depression of serum PTH by intravenous cab cium infusion in normal subjects. All subjects had normal base line calcium and PTH levels. Definite hypercalcemia developed in all patients during infusions.

ectopic PTH production was suggested during a one year follow-up.

Interpretation of PTH assay results in general has been rendered uncertain by the data of Ber- son and Yalow [6] suggesting immunologic hetero- geneity of circulating PTH. Arnaud and associates [7] have recently confirmed and extended their results. Limited information suggests that our antibody recognizes a circulating PTH that is immunologically distinct from highly purified PTH. In Figure 2 is shown a comparison of standard curves obtained with highly purified bovine PTH and partially purified human PTH, respectively. The scales on the abscissa could be adjusted to make the curves virtually superimposable. The

human PTH, obtained through the courtesy of Dr. J. T. Potts, was the same preparation as that reported by O’Riordan and associates [8]. Ap- propriate calculations indicated that our antibody possessed 1/10 the affinity for human hormone as it did for bovine hormone. Estimation of normal concentration of circulating PTH in our system

yielded values of approximately 30 mpg/ml. This is far in excess of the usual concentration of cir- culating peptide hormones [9]. It also is an order of magnitude higher than the reported normal concentration, approximately 1 mpg/ml, in the cow [10-J.

The second peculiarity of our assay concerns

the rate of hormone disappearance from the cir- culation when hypercalcemia is induced by cal-

cium infusions (Figure 3). The apparent half-time of hormone disappearance in these experiments was approximately three hours. This must be com- pared with an apparent half-time of twenty-eight minutes in the cow [ll]. Since the turnover of most peptide hormones is rapid, the discrepancy between our data and those obtained in the cow is not likely to represent a species difference.

In summary, our antibody appears to possess a remarkably high capacity for discriminating be- tween normal and abnormal concentrations of PTH under steady-state conditions. In the future, it is likely that well defined clinical material will have to be examined Iwith a variety of antibodies. There is thus hope of learning exactly what is being measured in various systems. For the pres- ent, we are encouraged by the excellent clinico- pathologic correlations obtained in primary para- thyroid disease.

SPECTRUM OF SECONDARY HYPERPARATHYROIDISM

It is now established that hypocalcemia [11,12] and hypomagnesemia [13] stimulate PTH secre- tion. Any clinical circumstance that decreases these divalent cations should lead to secondary hyperparathyroidism. Of the many forms of secondary hyperparathyroidism, the character- istic hyperparathyroidism of advancing renal fail-

GFR +

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P4 I

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Figure 4. Theoretical model of early renal hyperpara- thyroidism. Items enclosed within rectangles represent established observations in human chronic renal disease and in an experimental model studied in the dog. The intermediate steps of. a tendency toward hyperphos- phatemia and hypocalcemia are conjectural, but circum- stantial evidence favors their occurrence.

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ure has now been studied in some detail. It ex- emplifies the complexity and subtlety of the con- trol mechanisms in the intact organism.

A theoretic model was formulated some years ago on the basis of characteristic patterns of renal phosphate excretion in renal disease

[14,15]. The theory has been extensively elabor- ated by the experiments of Slatopolsky and associates [16-181. More direct confirmation has recently been obtained by applying the radioim- munoassay technic to studies of naturally occur- ring diseases in man and experimentally induced disease in dogs [4,5,19]. The theory is outlined in Figure 4. The key element of the theory is that, as the glomerular filtration rate decreases, tran- sient hyperphosphatemia occurs. This assumes no change in dietary phosphate intake and no gastro- intestinal or skeletal adaptations to the decreased renal excretion. According to this model, tran- sient hyperphosphatemia induces hypocalcemia which in turn stimulates PTH secretion. The end result of this adaptative sequence is a decreased glomerular filtration rate, normocalcemia and

Figure 5. Effect of oral phosphate on serum PTH, total calcium, ionized calcium and phosphorus in a normal man. Normal ranges: PTH, 10 to 60 ~1 Eq/ml; total calcium, 9.0 to 10.5 mg/lOO ml; ionized calcium, 4.08 to 4.80 mg/lOO ml; phosphorus, 3.0 to 4.5 mg/lOO ml. From Reiss et al. [20]. By permission of The Rocke- feller University Press.

normophosphatemia, the last two being obtained at the expense of some degree of hyperparathy- roidism. For this theory to be tenable, it must be demonstrated that (1) phosphate influences PTH secretion, (2) dietary phosphate affects the pat- tern of renal hyperparathyroidism and (3) serum PTH increases early in the course of progressive renal insufficiency. Some supporting data are now available for each of these points.

In Figure 5 are shown the results of the ad- ministration of 1 gm of phosphorus orally (as phosphate) to a normal volunteer subject. Serum PTH doubled, peak concentrations being obtained in approximately one hour. By two hours, serum PTH had returned to control values. A very small decrease of serum calcium, total and ionized, appeared to initiate this release of PTH. In fact, these changes were so small that one would ascribe them to technical error if a consistent pattern had not emerged in many experiments. That hypocalcemia rather than hyperphosphatemia triggered the increased PTH secretion was shown by failure of serum PTH to increase when calcium

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GENESIS OF HYPERPARATHYROIDISM- REISS. CANTERBURY

was infused at the same time that oral phosphate was given. Detailed analysis of this type of experi- ment is presented elsewhere [20]. For the pur- poses of this discussion, the exceptional sensi- tivity of the phosphate-initiated, calcium-mediated control system of PTH secretion is important.

Slatopolsky and associates [19] induced pro- gressive renal insufficiency in dogs by sequential ligation of branches of the renal arteries. One group of dogs had a normal phosphate intake; another group had a low phosphate intake (150 mg/day). The results of PTH measurements in these two groups are shown in Figure 6. When the phosphate intake was normal, serum PTH in- creased from normal to very high levels as the glomerular filtration rate decreased. By contrast, in dogs maintained on a low phosphate intake, hyperparathyroidism failed to develop. Thus, the dietary intake of phosphate clearly influenced the parathyroid response in this model.

In our experience, serum PTH increases very early in the course of renal insufficiency (Figure 7). Small decreases in glomerular filtration rate are associated with distinct increases in PTH.

The Slatopolsky-Bricker model is clearly ap- plicable only to the initiation of renal hyperpara- thyroidism. As renal disease advances, the com- plexity of the problem increases enormously. Hypocalcemia, induced by the vitamin D-resis- tant state that characterizes advanced renal fail- ure, is likely then to be the driving force for in- creased PTH secretion [21]. In fact, decreasing the serum phosphorus in advanced renal failure does not decrease serum PTH.

Renal hyperparathyroidism lends itself particu- larly well to exploration of the pathogenesis and sensitivity of various control mechanisms in dis- ease. If the sensitivity of the control of PTH secre- tion is as great as we have suggested, serum PTH should be increased in a great variety of human disease states. The parathyroid response in in- testinal malabsorption syndromes, a fertile field of investigation, has not been explored. We have obtained preliminary evidence of secondary hy- perparathyroidism in familial vitamin D-resistant rickets, renal tubular acidosis and acromegaly (un- published data). Systematic application of the radioimmunoassay to the study of larger popu- lations promises to uncover new instances of secondary hyperparathyroidism. The range of secondary hyperparathyroidism is probably very wide.

It has been assumed, since the studies of Patt

PTH

L J, 0

OR0 1

60 40 20 0 GFR ml/min

Figure 6. Progressive renal insufficiency was induced in dogs by sequential ligation of branches of renal arteries. From Slatopolsky et al. [19]. By permission of The Rocke- feller University Press.

and Luckhardt [12], that a low serum concen- tration of divalent cations is the sole stimulator of PTH secretion. This ‘dogma is now open to reinvestigation. Exploration of the possibility of other control systems is a major research effort in our laboratory.

PRIMARY HYPERPARATHYROIDISM

As routine measurements of serum calcium become increasingly common, primary hyper- parathyroidism is being diagnosed with increased frequency and probably at earlier stages of the disease. The diagnosis is now often made be- fore any symptoms are evident. Primary chief cell hyperplasia, which escaped detection until 1958 [22], is increasingly recognized and now consti-

Ir \ 0 IO 20 30 40 50 60 70 80 90 100

E CREATININE CLEARANCE ml/min

Figure 7. Relationship between creatinine clearance and PTH values in unselected patients with chronic renal in- sufficiency.

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Co Infusion 4 mg/kp/hr

-16 g

SZ -14 “\z

00 -12 “F

30 -- C

-‘O z I I I I 1 I 0 I 2 3 4 5

TIME-HOURS

Figure 8. Effect of calcium infusion on serum PTH in patients with parathyroid adenoma. Although some pa- tients were normocalcemic on the day of the infusion, all but one had had hypercalcemia at some time. Extension

of data previous/y reported [24].

tutes a substantial proportion of patients with primary hyperparathyroidism [23]. This patho- logic entity is particularly interesting because it may represent a precursor of parathyroid adenoma. In order to examine this possible re- lationship, it is necessary to study patients over long periods and to establish functional charac- teristics that distinguish hyperplasia from

16

TIME-HOURS Figure 9. Effect of calcium infusion on serum PTH in patients with primary chief cell hyperplasia. Extension of data previously reported [24].

684

adenoma. Unfortunately, histologic differentiation between hyperplasia and adenoma is often diffi- cult, if not impossible.

A fun,ctional differentiation is now possible by applying the radioimmunoassay in conjunction with intravenous calcium infusions [24]. Figure 8 shows results obtained by this technic in ten pa- tients who harbored solitary parathyroid ade- nomas. Despite the induction of severe hyper- calcemia, serum PTH remained constant. This is the pattern expected in an autonomously func- tioning neoplasm. By contrast, calcium infusion in patients with primary chief cell hyperplasia re- sulted in suppression of serum PTH in all but one patient (Figure 9). The patient showing failure of suppression was clinically and pathologically indistinguishable from the other patients with pri-

mary chief cell hyperplasia. Is it possible that he represents a transitional phase from suppressible hyperplasia to autonomous adenoma formation? Only long-term, repeated studies using these tech- nics will provide an answer. The hypothesis is attractive. It would provide a link between various forms of hyperparathyroidism. It would account for the frequency of reported instances of hyperplasia in patients diagnosed early. Against the hypothesis is the apparent rarity of autonomous hyperpara- thyroidism in patients with long-standing renal disease who are subjected to severe, chronic stimu- lation of the parathyroids [25]. Possibly, trans- formation from hyperplasia to adenoma is more likely to occur in very long-term stimulation than in more vigorous shorter-term stimulation. These speculations are not novel. The transition from secondary to primary hyperparathyroidism has been suggested, among others, by Dent’s clinical studies [26] and Jowsey’s careful histologic ex- amination of bone [27]. With the radioimmuno- assay, systematic investigation of this urgent clini- cal question is now possible.

The most vexing question concerning primary hyperplasia is, “What constitutes the driving force to the parathyroids?” In this connection it is interesting to recall the frequen,cy of primary chief cell hyperplasia in patients with the multiple endocrine adenomatosis syndrome [28]. It is now well recognized that this is a pleomorphic syn- drome that can occur with only one or two en- docrine systems demonstrably involved. The oc- currence of hyperparathyroidism in the syndrome raises a broader question of endocrine interrela- tionships of the parathyroids, a subject on which virtually no information exists. The tools are now at hand for studying the effects of the other pep- tide hormones on PTH secretion.

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REFERENCES

Volume 50, May 1971