Diagnosis of Growth Hormone Deficiency: the role of Growth Hormone (GH), Insulin-Like Growth Factor (IGF-I) and IGF-Binding Protein (IGFBP-3)

Since numerous reasons other than GH deficiency can cause growth retardation, the diagnosis of GH deficiency in a child with short stature should never be based solely on auxological or solely on biochemical- hormonal criteria. The diagnosis should be reached by a critical evaluation of all the relevant data brought together. In fact, even the normal limits for GH response to stimulation tests for endogenous GH secretion have not yet been fully standardised and this response is known to be influenced by factors like age, sex, pubertal stage and body mass index. Problems with the diagnosis of growth hormone deficency based on GH measurements alone are as follows:

1. The established approach to the diagnosis of GH deficiency is based on the assumption that GH deficient children have a lower GH response to stimulation compared to that of normally growing children. However, many studies have shown that the maximal response in normal children can remain below 10, 7 or even under 5 μg/L. Differences in methods used for measurement of GH in biological materials, interlaboratory differences in cutoff values for normal ranges arising from differences in methodology, difficulties inherent in the use of physiological stimulation tests as well as the multiplicity of agents used for pharmacological stimulation and the non-standardised use of sex steroids prior to the testing, all contribute to the present inadequacies of the diagnostic criteria.(1, 2, 3, 4)

2. Even if we all agree on a cut-off value, reproducibility of GH response on GH stimulation testing is poor. A significantly large proportion of the so-called GH deficient patients demonstrate normal response to GH re-testing after the completion of the treatment. In 25-44% of the individuals with the diagnosis of GH deficiency during childhood found to have normal GH levels on spontaneous GH secretion during sleep or on pharmacological testing when retested at adulthood.(5, 6)

3. Another area of imprecision comes from the question of conditions where the normalcy of GH secretion is evaluated (spontaneous vs stimulated). Although the best approach is considered to be the assessment of the spontaneous GH release, the difficulties associated with technicalities and the standardization of the results in this method as well as the costs, have restricted its application to research procedures only.(7, 8)

4. GH estimation in the body fluids is presently carried out by a variety of methods made available with commercial kits. These methods include radioimmunoassay (RIA) which utilizes polyclonal antibodies or enzyme-linked immunoabsorbant assay, immunoradiometric assay and ligand immunofunctional assay, all of which utilise monoclonal antibodies. A GH standard of 2 IU/mg, derived from hypophyseal tissue and made available by the ‘National Hormone and Pituitary Program’ (USA), used in the earlier stages of laboratory measurement of GH was later replaced by another standard of 2.6 IU/mg, also of hypophysial origin and coded as IRP 80/505. In 1998, WHO recommended the use of recombinant 22- kDa hGH (human GH), coded 88/624, of 3 IU/mg strength, and inclusion of this standard material in the kits presented to the market by the industry.(1, 2)

Although these new assays have provided sensitivity, speed, cost reduction and the facility of automisation, they have also brought the endocrinologist face to face with a number of problems. The use of polyclonal antibodies enabled the detection of all immunogenic epitopes of GH in the circulation, while the more recent assay kits with monoclonal antibodies, which do not recognize all components of GH in the circulation, give lower results for GH levels and necessitate the establishment of new and lower cutoff (threshold) values. For example, the previously accepted threshold of 5-7 μg/L should now become <5 μg/L. However, exactly the opposite has been recommended and the threshold has been raised to 10 μg/L. We believe this is an error and should be corrected. The new assays should be assessed on samples from normally growing children to establish the appropriate threshold levels.

Various pharmacological agents are being used for GH stimulation tests. The sensitivity of these tests is low.(4) The results obtained with these pharmacological agents onhealthy children of normal or subnormal height, using 7 μg/L and 10 μg/L as the lower limits of normal are shown in Table 1. The figures indicate the proportion of healthy children which show values under these cutoff limits. When the lower limit for GH deficiency is taken as 7.5 μg/L, the sensitivity is calculated as 73%, the specificity 83%, and the positive predictive value is 50%.

Stimulation of GH secretion with sex steroids: The physiological decrease in GH secretion just before puberty causes difficulty in differentiating this physiological event from GH deficiency. Therefore, exogenous sex steroids has been recommended to be used as a priming tool prior to the stimulation tests.(9) Priming with sex steroids can be done at age 10 in prepubertal girls and at age 12 in prepubertal boys. The use of ethynyl oestradiol can be recommended in both sexes at a single dose of 20-40 μg/day, given the night before the test. An alternative approach would be to administer 20 μg/day ´ 3 days ethynyl oestradiol in girls, and 100 mg i.m. testosterone enanthate in boys in one dose three days before the test. However, a consensus has not been reached regarding the use of exogenous sex steroids or the age at which they should be given. More data especially on final height is needed to clarify whether priming is helpful in differentiating those children who are not “really GH deficient” and therefore will not benefit from GH treatment.

WHICH PATIENTS SHOLUD BE TESTED?

For the clinical diagnosis of GH deficiency, after the elimination of skeletal dysplasias, genetic diseases such as Turner syndrome, other endocrinopathies, any chronic or systemic condition that might explain shortness or growth retardation, the history, physical findings and auxological data given below can be taken as signs of GH deficiency.(1)

Clinical findings suggestive of GH deficiency

Presence of (1) a family history of GH deficiency or of close consanguinity between the parents; (2) a history of perinatal trauma or of hypoglycemia, prolonged jaundice, micropenis in the newborn period; (3) anomalies such as the midline defects; (4) a history of cranial irradiation, intracranial lesions, head trauma, central nervous system infections and multiple hypophyseal hormone deficiency, have been agreed upon to be suggestive of GH deficiency.

Auxological findings suggestive of GH deficiency

Auxological findings accepted by the Growth Hormone Society (GHS) and the European Society of Paediatric Endocrinology (ESPE) as suggestive of a diagnosis of GH deficiency and which need to be confirmed by further investigation include:

1. Extreme shortness (height for age < -3 SD) without an explanatory reason

2. Medium shortness (height for age between -2 SD and -3 SD) with

a) a growth velocity less than 25th percentile or, in children > 2 years of age, a decrease of > 0.5 SDS in height noted after one year of follow-upb) a predicted height value lower than the target height by 1.5 SD (approximately 9-10 cm)

3. Without shortness of stature, a slow growth velocity of (< 2 SD or < 5 p) over 1 year or of < 1.5 SD over two years.

Estimation of target height

There are two equations which can be used for this calculation:

1. Target Height for Boys = (Mother’s Height+Father’s Height)/2+6.5 Target Height for Girls = (Mother’s Height+Father’s Height)/2–6.5

2. Midparental Height SDS = (Mother’s Height SDS+Father’s Height SDS)/1.6

DIAGNOSIS OF GH DEFICIENCY IN THE NEWBORN

In both preterm and term newborns, GH levels in the first days of life are >20 μg/L.10-11 IGF-I values, are relatively low, but it has been shown that in infancy, IGF-I values lower than -2 SD for age are suggestive of GH deficiency. In newborns with micropenis, hypoglycemia, birth trauma or a family history of GH deficiency, a value of 20 μg/L or below by routine GH estimation (using polyclonal antibodies) can be accepted as indicative of GH deficiency.

IGF-I AND IGFBP-3 IN GH DEFICIENCY

Changes in the levels of growth factors with age

In healthy children serum IGF-I and IGFBP-3 levels well reflect the endogenous 24-hour GH secretion. These levels have been recognised as useful clinical parameters since they show very little diurnal change and remain stable.(12, 13, 14, 15, 16, 17, 18, 19)

Insulin and IGF-I are the two main factors responsible for growth in the early postnatal period. Studies have shown that birth weight, placental weight and gestational age correlate positively with the cord blood IGF-I levels. In preterms, cord blood IGF-I, IGFBP-3 and the acid-labile subunit (ALS) levels are lower than in term newborns.(10, 20, 21)

In the early neonatal period, serum IGF-I levels are closely associated with nutritional state. This association weakens but remains through childhood and adulthood. The levels fall during periods of inadequate nutrition and rise with reversion to normal nutrition.(22)

The fundamental factors influencing growth are nutrition during the infancy period, primarily GH and also other hormones during childhood, and in addition to these, sex steroids during the pubertal period. In late infancy and early childhood, serum IGF-I and IGFBP-3 levels become GH dependent.

The relatively low serum IGF-I and IGFBP-3 levels at birth start increasing during childhood and reach maximal levels in adolescence and fall thereafter to prepubertal levels in adulthood.(23)

During childhood IGF-I levels increase slowly and show each year a parallelism with the growth rate of the following year.(23, 24) Longitudinal studies have shown that serum IGF-levels maintain this parallelism until the attainment of the maximal growth velocity. Thereafter, despite the fall in postpubertal growth rate, IGF-I levels have been observed to remain high. Therefore, the correlation between the IGFI levels and the growth rate is marked only in prepubertal children.(25, 26)

In both sexes, the increase in sex steroids during puberty results in higher GH secretion. Alongside with this increase in GH secretion, an increase in GH sensitivity also contributes to the increase in IGF-I and IGFBP-3 levels.(13) Peak IGF-I and IGFBP-3 levels are reached approximately 2 years after the attainment of peak height velocity.(27, 28)

Although serum IGF-I and IGFBP-3 determinations are very useful in evaluating growth disorders, a reliable normative data is needed for optimal benefit of these diagnostic tools. Since IGF-I levels vary with age, sex and puberty, a large sample is needed to determine normative values. There are differences in reported reference values for normal ranges due to differences in populations studied and differences in assay methods used.

Values obtained in a study using the immunoradiometric assay (IRMA) method (DSL assay kits) carried out on healthy school children in Istanbul in order to determine the reference range for IGF-I levels in Turkey are presented in Tables 2-7 and Figures 2-5 below. These data have made possible the calculation of IGF-I SDS and IGFBP-3 SDS values.(28, 29)

THE DIAGNOSTIC SIGNIFICANCE OF IGF-I AND IGFBP-3

In the diagnosis of GH deficiency

The conditions which affect the GH-IGF axis and the relative changes of serum GH, IGF-I ve IGFBP-3 levels are given in Table 8. Low IGF-I levels in GH deficiency have been shown in many studies. While IGF-I levels were found to be lower than -2 SD of the mean value for respective age in 82% of patients with GH deficiency, these values were found to be within normal limits in 68% of short children with no GH deficiency.( 30) In 203 boys of low stature, Juul et al.(31) have found that IGF- I values of - 2 SD below the mean had a positive predictive value of 57%. In children younger than 10 years, IGF-I estimation is more useful than estimation of GH in pointing to subnormality in GH stimulation tests (Figure 13). An IGF-I value of -2.5 SD gives the optimal limit for discriminating GH deficiency from idiopathic low stature.(31)

The diagnostic value of estimating IGF-I and IGFBP-3 changes according to the cutoff limits taken, Rikken et al.(32) have demonstrated in a study with 96 children that when a cut-off limit of -0.83 SD was taken for IGF-I, the sensitivity and the specificity of the estimations for detecting GH deficiency were 92% and 47%, respectively. If the IGF-I cut-off limit was taken as -1 SD, the percentages of GH deficiency and idiopathic low stature detected became 88 and 46, respectively. In other studies taking the IGF-I limit at -2 SD, the specificity and the sensitivity of detecting GH deficiency varied between 47% and 80% and 61% and 91%, respectively.(33, 34, 35, 36, 37, 38, 39, 40)

Blum et al. in 1990 have stated that the sensitivity and the specificity of measuring IGFBP-3 were 97% and 95%, respectively, and that estimating IGFBP-3 was more useful than measuring IGF-I for the diagnosis of GH deficiency. However, in subsequent studies these high levels of specificity and sensitivity were not demonstrable and the reported sensitivities varied between 15% and 98% and the corresponding specificities varied between 50% and 98%.(42, 43, 44) After the publication of the reference curves for IGF-I and IGFBP-3 values over the age range 0 to 6 years, it was seen that the lower limits of IGF-I levels were very close to the estimated IGF-I values whereas this was not the case with IGFBP-3. Thus, theoretically IGFBP-3 has a diagnostic superiority over IGF-I for the 0 to 6-year age group and this hypothesis has been supported by other studies demonstrating the diagnostic superiority of IGFBP-3 estimations in prepubertal children as compared to those in older children.(24, 31, 33)

In most patients with GH deficiency IGFI and IGFBP-3 levels are low and rise to normal after treatment. IGF-I and IGFBP-3 can be used in evaluation of response to treatment as well as in the follow-up of GH deficient patients, regardless of etiology. The follow up of IGF-I and IGFBP-3 levels theoretically will help to predict the growth response as well as assessing the efficacy of GH replacement and patient compliance to the treatment. In patients receiving GH replacement, positive correlations between the z-scores of IGF-I and IGFBP-3 and the increase in height have been shown. In GH replacement dose adjustment IGF-I values specific for age and sex must be taken into consideration.

In recent years, it has been argued that both from the points of view of effectiveness and long term safety, the adjustment of the GH replacement dose be made according to IGF-I and IGFBP-3 values.(45) The advantages and disadvantages of the criteria used in adjusting the GH replacement dose are shown in Table 10.

The targeted IGF-I z-scores in different stages of the replacement therapy for optimal benefits are shown in Table 11.

Corresponding Author: Abdullah Bereket, Marmara University Medical School, Pediatric Endocrinology Department, Istanbul, Turkey E-mail: abereket@e-kolay.net