Bioactive GH-like immunoglobulins G in active acromegaly: response to long-term treatment with bromocriptine

Laboratory measurement of growth hormone

Growth hormone (GH) measurements are complicated by the heterogeneous nature of GH, as well as by the presence of the GH binding protein in plasma. Several isoforms of GH exist, and specific assays for each are currently either unavailable, impractical, or not clinically indicated. Bioassays include the in vivo assays based on rat weight gain, tibial line widening, or IGF-I generation. In vitro bioassays, based on the proliferation of cell lines expressing the prolactin receptor or GH receptor, are sensitive but prone to nonspecific interference by factors present in serum. Immunoassays (RIA, IRMA, ELISA, and immunofunctional assay design) are widely used in the clinical laboratory because of speed, sensitivity, and convenience. Discrepancies among results rendered by different immunoassays have become more apparent as monoclonal assays have superseded polyclonal assays, presumably because different antibodies recognize different epitopes among the heterogeneous mixture of GH isoforms in serum. Some assays, especially those with short, nonequilibrium incubation times are vulnerable to interference by the GH binding protein present in serum. Recommendations are given for strategies designed to minimize disparity of results obtained by different GH immunoassays applied to serum. Urinary GH measurements, while technically feasible, are of limited clinical utility because of biological variation in urinary GH excretion.

The effect of treatment with the oral growth hormone (GH) secretagogue MK-677 on GH isoforms

Growth hormone (GH) consists of several isoforms. We have studied the proportion, expressed as percentage of total GH concentration, of non-22kDa (non-22K) GH isoforms and 20K GH during 8-week oral treatment with MK-677 25mg daily in 12 obese males. The proportion of non-22K GH isoforms in peak total GH samples after the initial MK-677 administration was higher than that after 2 and 8 weeks (p<0.01 and p<0.05, respectively). In selected non-peak total GH samples after the initial MK-677 administration, however, the proportion of non-22K GH isoforms was similar to that in the peak total GH samples after 2 and 8 weeks. The proportion of 20K GH in 2-h samples after the initial MK-677 administration was lower than that after 2 and 8 weeks (p<0.01 and p<0.05, respectively). We concluded that the proportion of non-22K GH isoforms was higher in peak, but not in non-peak, total GH samples after the initial MK-677 administration than that observed after multiple doses. The proportion of 20K GH in 2-h samples after the initial MK-677 administration was lower than that after 2 and 8 weeks. These moderate changes in the proportion non-22K GH isoforms are likely of small importance for the clinical response to MK-677 treatment.

O Laboratório no Diagnóstico e Seguimento da Acromegalia

Acromegalia é uma síndrome clínica característica, que na maior parte das vezes resulta de um macroadenoma hipofisário produtor de hormônio de crescimento (GH, growth hormone). A hipersecreção tumoral crônica de GH provoca deformidades esqueléticas, distúrbios metabólicos, complicações em vários órgãos e sistemas, e acaba reduzindo a expectativa de vida do paciente. O diagnóstico presuntivo baseia-se nos achados clínicos característicos da doença, com a confirmação vindo através de exames laboratoriais e da avaliação radiológica. Do ponto de vista laboratorial, a abordagem diagnóstica inclui dosagens basais e testes endócrinos que comprovem o excesso de GH, através de dosagens diretas do GH e/ou de fatores circulantes GH-dependentes, cujo melhor exemplo é o fator de crescimento insulina-símile-1 (IGF-1, insulin-like growth factor-1). A adenomectomia transesfenoidal permanece como o tratamento inicial de escolha na acromegalia, mas infelizmente a cura cirúrgica ocorre em menos da metade dos pacientes portadores de macroadenomas. Conseqüentemente, os exames laboratoriais têm um papel muito importante no seguimento dos pacientes após a cirurgia, para definir os critérios de cura e monitorar a atividade da doença durante tratamento complementar com radioterapia ou medicamentos. Neste artigo, revisaremos os exames laboratoriais mais freqüentemente utilizados no diagnóstico da acromegalia e alguns métodos experimentais que vêm sendo testados na sua abordagem. Na parte final, apresentaremos as principais recomendações de dois workshops internacionais realizados nos últimos anos com o objetivo de padronizar a avaliação diagnóstica e a conduta terapêutica na acromegalia.