Spontaneously occurring antibodies to parathyroid hormone

Primary hyperparathyroidism with a low-normal, atypical serum parathyroid hormone as shown by discordant immunoassay curves

CONTEXT

In patients with primary hyperparathyroidism (PHP), one expects to find a serum PTH in the high or high-normal range. The presence of a low-normal PTH in PHP can be difficult to explain.

OBJECTIVE

Our objective was to investigate the cause of a low-normal serum PTH in a patient with PHP.

PATIENT

A 57-yr-old asymptomatic white female from the private practice of F.W.L. presented with an 8-yr history of a rising serum calcium from 10.5-11.6 mg/dl (2.63-2.88 mmol/liter) and a low-normal serum intact PTH of 29.2 pg/ml. After localization of a parathyroid adenoma by [(18)F]fluorodesoxyglucose positron emission tomography scanning, a 120-mg parathyroid adenoma was removed with the achievement of normocalcemia for the subsequent 2 yr.

METHODS

Routine pre- and postoperative serum intact PTH assays were preformed at both the Quest Diagnostics regional laboratory in Pittsburgh, Pennsylvania, and at the Quest Diagnostics Nichols Institute in California. In addition, intact, biointact, and C-terminal assays were measured in undiluted, 1:2 diluted, and 1:4 diluted sera at the Nichols Institute. PTH gene sequence analysis was performed from DNA extracted both from the parathyroid adenoma and the patient's peripheral blood leukocytes.

RESULTS

Dilution, with correction for the dilution factor, of the preoperative serum produced a progressive rise in the intact, biointact, and the C-terminal assays, whereas no dilution effect was seen in postoperative serum. No intragenic mutations in the pre-pro-PTH coding region were found in either the parathyroid adenoma or matched blood DNA samples.

CONCLUSIONS

The discordant preoperative immunoassay curves with dilution could not be explained by the adenoma producing a mutated PTH. Furthermore, an autoantibody against the PTH produced by the adenoma is ruled out by the prompt loss of the dilution effect in the three PTH assays within 1 wk of the adenoma's excision. A posttranslational effect on the PTH molecule within the adenoma remains a possible explanation for the discordant immunoassay curves. Our report emphasizes that one cannot always rule out PHP because of a low-normal serum intact or biointact PTH. Repeated PTH measurements after serum dilution in suspected cases of PHP with low-normal PTH levels may be a useful method for detecting atypical forms of PTH.

Parathyroid and autoimmunity

Data in favor of chronic hypoparathyroidism as an autoimmune disease are examined. The article takes into consideration the different clinical forms, genetic patterns, histopathology, animal models, cellular immunity, circulating autoantibodies, target autoantigens, clinical manifestations, laboratory diagnosis and therapy. Furthermore, data on 71 Italian patients with chronic hypoparathyroidism are presented.

Spontaneously occurring anti-PTH autoantibodies must be considered in the differential diagnosis of patients with elevated serum PTH levels

We investigated the cause of elevated immunoreactive circulating parathyroid hormone (PTH) levels in two females of 41 (case 1) and 39 (case 2) years of age with low/normal serum calcium levels and hypocalcemia, respectively, and, in the latter case, hyperprolactinemia. Serum samples from both patients were fractionated by Sephadex G-100 Superfine chromatography. Fractions were assayed for PTH and prolactin (PRL) by immunoradiometric assays (IRMA) and for immunoglobulin G (IgG) by radial immunodiffusion. Sera from both patients were incubated with protein A and protein G Sepharose, centrifugated and the supernatant was assayed for PTH by IRMA. Sera were also subjected to affinity chromatography with an anti-human-IgG-agarose column. IgG and PTH or PRL were measured in the fractions by radial immunodiffusion and IRMA, respectively. In both cases the majority of serum PTH immunoreactivity eluted in the same fractions of IgG after gel filtration and was precipitated by protein A (89% in patient 1 and 96% in patient 2) and protein G (83% in patient 1 and 100% in patient 2), thus, behaving as IgG. In case 1, 79% of PTH was also retained by an anti-hIgG agarose column. High PRL levels in patient 2 were due to macroprolactinemia since most of PRL eluted as big,big (40%) and big-PRL (45%) after gel filtration. Forty-eight percent of PRL from patient 2 was retained by the anti-hIgG column indicating the presence of an anti-PRL autoantibody. These data suggest that spontaneously occurring anti-PTH autoantibodies must be considered in the differential diagnosis of patients with elevated serum PTH levels.

Neutralizing interferon beta antibodies in melanoma patients treated with recombinant and natural interferon beta

The incidence and clinical significance of therapy-induced neutralizing interferon beta (IFN beta) antibodies was studied in a group of 21 melanoma patients treated with natural IFN beta and 7 patients treated with recombinant IFN beta. They were treated subcutaneously with 3 x 10(6) IU three times per week in an adjuvant open trial for 24 weeks after surgical removal of all detectable metastases. Of the 21 patients treated with natural IFN beta, 95% developed significant levels of neutralizing antibodies after 24 weeks. In comparison, 28% of the 7 patients treated with recombinant IFN beta developed neutralizing IFN beta antibodies. Cross-reactivity of the antibodies could be demonstrated. Persistence of antibody titers was seen in 80% of the patients 24 weeks after cessation of treatment with natural IFN beta. No correlation between the maximum antibody titers and the antibody persistence after cessation of therapy could be established. We detected a clear correlation between the formation of neutralizing antibodies and the decrease in beta 2-microglobulin and 2',5'-oligoadenylate synthetase and therefore the drop in biological activity. In this adjuvant trial there was no difference in relapse rate and time until relapse between antibody-positive and antibody-negative patients. No difference in clinical outcome could be established between the patients treated with natural IFN beta and recombinant IFN beta.

Antiidiotypic PTH antibodies as a cause of elevated immunoreactive parathyroid hormone in idiopathic hypoparathyroidism, a second case: another manifestation of autoimmune endocrine disease?

A 69-year-old man became hypocalcemic under medical observation. The hypocalcemia occurred in the presence of circulating immunoreactive parathyroid hormone (PTH). Common causes of secondary hyperparathyroidism were excluded, as was PTH resistance using PTH infusions. The immunoreactive PTH was examined in detail. PTH immunoreactivity (1) was not retained on a C18 SPE-column, suggesting unusual molecular or physicochemical properties, unlike bona fide PTH; (2) was precipitated with 15% PEG, indicating a molecular size far in excess of native PTH; (3) had an apparent molecular size similar to immunoglobulins on size exclusion chromatography; (4) was retained on affinity chromatography with both Protein A and anti-hIgG antibodies. These data lead us to conclude that the immunoreactive PTH was due to antiidiotypic PTH autoantibodies. No significant quantities of true PTH were found in the patient's serum suggesting that his hypoparathyroidism was a result of PTH deficiency. Autoimmunity might explain the occurrence of both processes if an arrested antiidiotypic antibody cascade is assumed.