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Diagnosis of Growth Hormone (GH) Deficiency in Adults with Hypothalamic-Pituitary Disorders: Comparison of Test Results Using Pyridostigmine Plus GH-Releasing Hormone (GHRH), Clonidine Plus GHRH, and Insulin-Induced Hypoglycemia as GH Secretagogues¹

Department of Endocrinology and Medicine, Aalborg Hospital, DK-9000 Aalborg, Denmark

Address all correspondence and requests for reprints to: Hans C. Hoeck, M.D., Ph.D., Department of Endocrinology and Medicine, Aalborg Hospital, Reberbansgade, DK-9000 Aalborg, Denmark. E-mail: hans.chr.hoeck{at}dadlnet.dk

	Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
The insulin tolerance test (ITT) is widely accepted as the method of choice to evaluate GH secretion capacity in adults with hypothalamic-pituitary disorders. However, the test is not suitable in the elderly or in patients with cardiovascular disease or seizure disorders. In recent years alternatives to the ITT have been introduced. The purpose of the present study was to investigate the diagnostic outcome with the ITT, the pyridostigmine plus GHRH (PD+GHRH) test, the clonidine plus GHRH (CLO+GHRH) test, and insulin-like growth factor I (IGF-I) in an unselected group of patients with hypothalamic-pituitary disease. An evaluation of the reproducibility of the different stimulation tests was included in the study. Based on repeated testing with the various GH stimulation tests in healthy adult males and females, the lower limits of normality for the ITT, the PD+GHRH test, and the CLO+GHRH test were 3.92, 12.8, and 19.0 µg/L, respectively. A consecutive group of 26 unselected patients with hypothalamic-pituitary disorders, 13 males and 13 females (median age, 44 ys), were tested twice with all stimulation tests, except that only 10 patients were tested once with the CLO+GHRH test due to side-effects related to clonidine. The peak GH responses between test 1 and test 2 correlated significantly in both the ITT and the PD+GHRH test (P < 0.02), and no significant difference was observed in the median peak response to repeated testing. In addition, no sex difference was observed. The coefficients of variation (CV) were 96% (ITT) and 45% (PD+GHRH), but in the majority of patients low values were repeatedly low. The peak GH response was significantly higher during the PD+GHRH test than during the ITT (P = 0.008). In the 10 patients tested with the PD+GHRH and CLO+GHRH tests there was no significant difference in the peak GH response (P = 0.398). When the test specific cut-off values were used, no significant difference in diagnostic outcome was observed between the various tests (P > 0.3). In contrast, the diagnosis obtained with IGF-I differed significantly from all GH stimulation tests (P < 0.03). Twenty (77%) and 22 (85%) patients were diagnosed to be GH deficient with the ITT and the PD+GHRH test, respectively. Of the 14 patients with multiple pituitary failure (>2 hormones affected), GH deficiency was present in more than 90% regardless of the type of stimulation test used. The IGF-I levels were only subnormal in 42% of the patients and did not correlate with the peak GH responses in any of the stimulation tests (P > 0.05). Except for 1 patient all patients with subnormal IGF-I were GH deficient in all stimulation tests. It is concluded that in patients with hypothalamic-pituitary disease and a normal IGF-I level 2 stimulation tests should be performed to establish a diagnosis of GH deficiency. In patients with a subnormal IGF-I value a single GH stimulation test should be sufficient to confirm the presence of GH deficiency.

	Introduction

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SEVERAL STUDIES in groups of adult patients with hypothalamic-pituitary disorders and severe GH deficiency have demonstrated clear benefits from substitution therapy with recombinant human GH (1, 2, 3, 4). However, the clinical characteristics of the GH deficiency syndrome can be vague in the individual patient (5). A single measurement of GH in serum is useless for diagnosing GH deficiency, and evaluation includes provocative testing (6, 7). The unspecific and variable responses in healthy adults to GH stimulation tests are well recognized (8, 9, 10, 11), but other approaches to measure GH secretion capacity in adults have been disappointing (12, 13, 14, 15).

The insulin tolerance test (ITT) has been shown to be useful in pituitary patients with multiple hormone deficiencies (12, 13, 16). However, the ITT may be unpleasant to the patient and is contraindicated in the presence of cardiovascular disease or seizure disorder. Alternative stimulation tests have been used in pediatric endocrinology for decades (17, 18), but some have proven to be less useful in adults (19, 20).

In recent years new GH stimulation tests have been introduced (10, 13, 19, 21, 22). Some of the tests have fewer side-effects and are more potent stimulators of GH secretion in healthy adults compared to the ITT. However, uncertainty remains to what extent the same diagnosis is obtained with the different GH stimulation tests in the same patient. This was investigated in the present study with three different GH stimulation tests in an unselected group of patients with hypothalamic-pituitary disease. The diagnosis of GH deficiency was compared when using either a fixed cut-off value or a test-specific cut-off value calculated from test results in healthy adults (11). In addition, the reproducibility of the tests was investigated.

	Subjects and Methods

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Twenty-six patients, 13 males and 13 females, with hypothalamic-pituitary disorders and variable degrees of hypopituitarism (all receiving stable substitution therapy where appropriate, including substitution with gonadal hormones) were tested twice with the ITT and the pyridostigmine plus GHRH (PD+GHRH) test. In addition, 10 of the patients (5 males and 5 females) were tested once with the clonidine plus GHRH (CLO+GHRH) test. There was a minimum of 72 h between the tests. The patients were recruited consecutively at their routine visits in the department and finally by asking only females so as to obtain an equal number of males and females. None of the patients had received GH substitution therapy in childhood. The individual characteristics of the patients are listed in Table 1.

The clinical studies were mainly performed in an ambulatory setting, i.e. the participants were asked not to eat after 2000 h the previous evening and to avoid major physical activity and shower bath before arrival. The participants were asked to be transported by car or bus.

All subjects were tested in the supine position. An indwelling heparin-locked cannula was inserted in an antecubital vein. The subjects then rested for 30 min before initiating one of the test procedures.

The ITT

The test procedure was the same in all patients, except that some subjects were tested under in-house conditions depending on the preference of the patient. This did not affect the results, as no significant difference was observed between the peak GH responses of the 2 tests in the 14 patients in whom 1 of the tests was performed in an ambulatory setting (P = 0.79). Blood samples were drawn for the determination of glucose and GH before insulin (Insulin Actrapid Human, Novo Nordic, Copenhagen, Denmark; 0.15 IU/kg BW) was injected iv. Blood glucose was measured with Reflolux (Roche, Mannheim, Germany) with short intervals. When blood glucose had declined to 2.2 mmol/L or less, blood samples were collected for measurements of GH and glucose (glucose oxidase method). Thereafter, blood samples were drawn 30 and 60 min later for determination of GH and glucose. In all subjects a decline in blood glucose of 2.2 mmol/L or less (median, 1.7 mmol/L) was observed during the ITT with the standard dose of insulin. Five minutes after adequate hypoglycemia had been observed and blood samples had been taken, 50 g glucose in an aqueous solution were given orally over a period of 10 min to prevent late hypoglycemic episodes. In none of the patients were serious side-effects, necessitating the infusion of glucose, reported.

The CLO+GHRH test

Before starting the test, blood samples for measurements of GH were collected, and 30 min later clonidine (Catapresan, Boehringer Ingelheim GmbH, Ingelheim, Germany; 300 µg) was given orally with a glass of water. Blood samples for measurement of GH were collected 15, 30, 45, and 60 min later. Then GHRH-(1–29)-NH₂ (Groliberin, Pharmacia & Upjohn, Inc., Stockholm, Sweden; 1 µg/kg BW) was injected iv, and blood samples for measurement of GH were drawn at 15, 30, 45, 60, 75, and 90 min after the injection of GHRH. The test was performed in 10 patients and only once, as the adverse effects to clonidine (extreme drowsiness, dryness of the eyes and mouth) were so unpleasant that further studies with this test was omitted.

The PD+GHRH test

Blood samples for measurement of GH were drawn. Thirty minutes later pyridostigmine (Mestinon, Roche; 120 mg) was given orally with a glass of water. Blood samples for measurement of GH were collected 15, 30, 45, and 60 min later. Then GHRH-(1–29)-NH₂ (Groliberin, Pharmacia & Upjohn, Inc.) or GHRH-(1–44) [Somatrel hGRF-(1–44), Ferring Pharmaceuticals Ltd., Copenhagen, Denmark; 1 µg/kg BW] was injected iv, and blood samples for measurement of GH were drawn at 15, 30, 45, 60, 75, and 90 min after the injection of GHRH. In general, the PD+GHRH test was well tolerated in all patients. Only mild gastrointestinal discomfort was reported in a few subjects.

Glucose and GH (immunoradiometric assay from CIS-Bio International, Gif-sur-Yvette, France) were measured as previously described (23). Insulin-like growth factor I (IGF-I) was measured on a fasting blood sample using a noncompetitive immunoradiometric assay (Diagnostics Systems Laboratories, Inc., Webster, TX) as previously described (11).

Informed written consent was obtained from all participants, and the study protocols were approved by the scientific ethical committee of Viborg and Nordjylland County.

The statistical evaluation was carried out using nonparametric tests, including Spearman’s rank correlation coefficient, Wilcoxon’s ranking test for unpaired data, Fisher’s exact test, and the ² test. P < 0.05 was considered to be statistically significant. The coefficient of variation was calculated from CV = [((t1 - t2)²)/(2 x n)]/mean, where t1 is the peak GH response in test 1, and t2 is the peak response in test 2.

	Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
The individual peak GH responses to all tests and the serum IGF-I values are shown in Table 2. In both the ITT and PD+GHRH tests there were no significant differences in the peak GH responses between tests 1 and 2 (P > 0.07). In addition, no sex difference was observed in either of the tests (P > 0.10). A significant correlation between the peak GH response in tests 1 and 2 was observed in both the ITT and the PD+GHRH test (P < 0.02). However, although the peak GH responses in the majority of patients were repeatedly low, the peak GH responses to repeated testing were highly variable in both the ITT and the PD+GHRH test, with coefficients of variation of 96% and 45%, respectively.

View larger version (24K): [in this window] [in a new window]   Figure 1. The diagnosis of GH deficiency using test-specific peak GH values. Comparisons of GH tests and IGF-I. The serum IGF-I and peak GH responses from the mean of tests 1 and 2 in 26 patients [13 males (•) and 13 females ()] during the ITT, the CLO+GHRH test, and the PD+GHRH test. A, Results in patients with only one or no other pituitary hormone affected; B, data from patients with multiple hormone deficiencies. The IGF-I values were logarithmically transformed and are shown as a percentage of the lower limit of the age-adjusted reference value. The peak GH values were logarithmically transformed and are presented as a percentage of the calculated lower 95% confidence limit from each test in the control subjects (11 ). The 95% confidence ranges are indicated by the bars and were calculated from the mean of the peak GH response in test 1 and test 2 in healthy adults after logarithmic transformation (11 ).

The IGF-I level was subnormal in 42% of the patients (Table 2 and Fig. 1). No correlation was observed between the IGF-I levels and the stimulated peak GH responses in any of the tests (P > 0.05). However, with a single exception, all subjects with a subnormal IGF-I had subnormal test responses with all GH stimulation tests (Fig. 1). In some of the patients the IGF-I levels were within the lower end of the age-adjusted reference interval, although the patients had a repeatedly blunted stimulated GH response. No statistically significant difference was found between the IGF-I values in patients with no other hormone deficiency and those with multiple hormone defects.

	Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
This study is the first to evaluate repeated testing with the ITT and the PD+GHRH test in a larger unselected group of patients with hypothalamic-pituitary disease. The stimulated peak GH responses were variable in all tests, but clear differences were observed. The range of peak GH responses was wider during the PD+GHRH test compared to the ITT, although the variability was lower. This observation could support the hypothesis that somatostatin tone has a major impact on the magnitude of the stimulated GH response (24). In general, the results obtained with the different tests indicate that GH secretion after injury to the hypothalamic-pituitary region is not just either present or absent, but ranges from normal down to values around the detection limit of most GH assays. However, despite a high variability to repeated testing, low GH values were reproducible in both the ITT and the PD+GHRH test, indicating the presence of prominent injury to the GH regulatory system in these patients. It is well recognized that adiposity impairs the magnitude of the GH response to provocative testing (25, 26). Sixteen of the 26 patients had a BMI above 25, which may bias the results, as the control group mainly consisted of lean subjects (11). Unfortunately, the CLO+GHRH test was associated with unpleasant side-effects, limiting the use of this test in a clinical setting.

The studies were performed with two different preparations of GHRH. Initially, GHRH-(1–29)-NH₂ was used. Based on previous studies the biological activity of human GHRH resides in the first 29 amino acids (27). The manufacturer ceased the production of GHRH-(1–29)-NH₂ during the study period, and we changed to GHRH-(1–44). As no significant differences have been observed between the two preparations on a weight basis, it is unlikely that this has introduced any significant bias to the results of the study (28, 29).

Similar to the observations in healthy adults, significant differences in the peak GH response to the different tests was observed (11). The PD+GHRH tests also provoked higher GH responses compared to the ITT. This difference was present when analyzing the 12 patients with deficiency of two or more pituitary hormones separately (test 1, P = 0.011; test 2, P = 0.001). Andersen et al. observed a similar significant difference in the peak GH response during the ITT and the PD+GHRH test (10). However, Ghigo et al. found almost identical results with the same tests in pituitary patients (13). The controversy is readily explained by differences in the characteristics and the number of the patients studied. In contrast to the study by Andersen et al. and the present study, the majority of the 11 patients in the study by Ghigo et al. had deficiencies of 3 pituitary hormones (10, 13). In the present study the degree of impaired GH secretion correlated with the degree of hypopituitarism in the PD+GHRH test. This observation is identical to findings with the ITT in a large group of pituitary patients (16).

A cut-off limit of 3 µg/L has been suggested for the ITT (7, 30), and a recent study observed no differences in the stimulated GH response in pituitary patients during the ITT and PD+GHRH test (13). The present data demonstrate major diagnostic differences as a consequence of using a fixed cut-off value of 3 µg/L in the three tests. In contrast, no systematic diagnostic differences between the tests employing the test-specific cut-off values calculated from repeated testing in healthy adults were observed.

Three patients had a low normal GH response in the ITT, but were diagnosed GH deficient in the PD+GHRH test. All of them had IGF-I levels in the normal range. In addition, one patient (patient 13) had a marked difference in the GH response, with a repeatedly subnormal response to the ITT and a low IGF-I, but a normal response to the PD+GHRH test. This patient had been treated with radiotherapy for a hypothalamic medulloblastoma. This might indicate that either the hypothalamus or the connection between the hypothalamus and the pituitary had been damaged and that the patient’s pituitary was intact. A direct pituitary stimulus using PD+GHRH could then release GH. In contrast, hypoglycemia may exert its GH-releasing effects at the hypothalamic level, and no GH release would occur.

In accordance with the observations by others (10, 12) the IGF-I levels did not correlate with the peak GH responses in any of the tests (ITT, P > 0.05; PD+GHRH, P > 0.1). Nearly all patients with subnormal IGF-I had a blunted peak GH response regardless of the type of test used, whereas some patients with normal IGF-I had very low responses to all tests. No statistically significant difference was found between the IGF-I values in patients with no other hormone deficiency and multiple hormone defects. This is in sharp contrast to the observations with the GH stimulation tests, where the amount of GH secreted correlated with the degree of hypopituitarism in all tests. These findings may suggest that the GH stimulation tests are better estimators of GH reserve, compared to IGF-I, in patients with hypothalamic-pituitary disorders.

The results of the present study have practical clinical implications in the evaluation of patients with hypothalamic-pituitary disease. In patients with subnormal IGF-I a single GH stimulation test seems sufficient to confirm the diagnosis. In patients with normal IGF-I it is suggested to perform two stimulation tests. Provided that test-specific cut-off values are used, the diagnostic value of the PD+GHRH test is equal to that of the ITT.

	Acknowledgments

 
The GHRH used in the studies was kindly provided by Pharmacia & Upjohn, Inc. (Groliberin), and Ferring Pharmaceuticals Ltd. A/S (Somatrel). We are grateful to Mrs. Ester Ditzel, Mrs. Anni Nielsen, Mrs. Marianne Køhler, and Mrs. Thea Kragh for their skillful assistance in conducting the experiments and in the analysis of the hormonal samples.

	Footnotes

 
¹ This work was supported by grants from Stiftstidendes Julelotteri, Det Obelske Familiefond, Kristen Tøfting og Dagmar Tøftings Fond, and Nordjyllands Lægekredsforenings Forskningsfond.

Received September 29, 1999.

Revised December 15, 1999.

Accepted December 23, 1999.

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