Growth Hormone Test

    
Testing Of IGF-1 for Growth Hormone

By Jay H Mead MD

At puberty, during the adolescent growth spurt, growth hormone (GH) levels increase and then decline slowly and remain stable until mid-adulthood. After this time levels began to decrease at approximately 14% per decade until about age 60 when they plateau. The physiologic decrease in GH is thought to be the primary factor in the aging process. Adult growth hormone deficiency can lead to rapid aging which includes a host of symptoms among them: reduced stamina, low sex drive and reduced function, decreases in bone density, muscle mass, cardiac function, blood sugar control, immunity and memory. Therefore, the pharmacological use of GH boosting agents has been proposed to ameliorate the metabolic derangements associated with absolute and relative GH deficiency.

Diagnosing GH deficiency and monitoring the therapeutic levels of GH replacement therapy is problematic. Because GH fluctuates radically throughout the day, random serum measurements are unreliable as diagnostic and therapeutic markers. Fortunately there is a stable biologic mediator of GH, Insulin-Like Growth Factor-1 (IGF-1), readily measurable in serum (Ref 6,7). IGF-1 or somatomedin C is a protein produced by the liver and released into the blood stream. IGF-1 production is controlled by GH release from the pituitary gland, which is in turn controlled by the brain through signals from the hypothalamus. IGF-1 is produced by the liver in response to the average GH level throughout the day.

IGF-1 is found in serum in free and bound forms. The bound form is complexed with an acid labile subunit (ALS) and a “binding protein.” There are six Insulin-like Growth Factor Binding Proteins (IGFBP) that facilitate the transport of IGF-1 in the blood stream. Of the six binding proteins, IGFBP-3 is the most important because it has the highest blood level and it’s synthesis is also controlled by GH. Testing for IGFBP-3 level is mainly of value in prepubescent children and is of very little benefit in evaluating Adult Growth Hormone Deficiency (AGHD) (Ref 1).

There is excellent correlation with GH release and extracted IGF-1 measurements from serum; therefore, IGF-1 measurement in the blood is an indirect method of assessing GH levels (Ref 1,5,7). As with any indirect measurement, it is imperative to keep in mind that other factors may influence the liver function and, therefore, modulate IGF-1 production. Other factors known to lower IGF-1 (Ref 2, 3,6) levels include oral estrogen, protein calorie malnutrition, insulin deficiency, liver failure, hypothyroidism, pregnancy, glucocorticoid therapy, renal failure and acute catabolic stress (eg, surgery, trauma, hip fractures and infectious diseases). All of these factors are thought to stress the liver and interfere with IGF-1 production.

Filter paper blood spot testing employed by ZRT Lab has shown to be as effective as serum for IGF-1 testing (Ref 7). ZRT Lab uses a “state of the art” immunoradiometric (IRMA) sandwich assay proven to exceptionally sensitive and specific for each analyte tested.

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