A 14.2-year-old prepubertal boy diagnosed with complete-type growth hormone deficiency and tertiary hypothyroidism, keeps growing in the height range between –1 and –2 SD. He has been treated with levothyroxine only. To understand the growth mechanism of this boy, we analyzed the serum growth hormone (GH) with a radioimmunoassay (RIA), serum GH bioactivity with Nb2 and erythroid progenitor cell bioassays, and growth hormone-binding protein (GHBP) with a ligand-mediated immunofunctional assay (LIFA). In addition, IGF-1 and free IGF-1 were analyzed by immunoradiometric assay (IRMA) and insulin-like growth factor-binding protein-3 (IGFBP-3) by Western immunoblot. Peak GH-RIA responses to insulin, arginine and GH-releasing factor, and nocturnal GH secretion, were low (0.5–2.3 ng/ml); bioactive GH was low (0.313 ng/ml), and GHBP was elevated (84 ng/ml). The serum levels of IGF-1 and free IGF-1 were continuously low, 17.1–39.3 and 0.17–0.26 ng/ml, respectively. Moreover, serum IGFBP-3 levels were low (1.68– 1.39 mg/l) and IGFBP-3 protease activity was negative. Prolactin and insulin were in the normal range. The result of the assay for growth-promoting activity showed that the patient’s serum stimulated normal erythroid progenitor cells twice as potently as did healthy thin adult control serum. These results suggest that GH and IGF-1 are not indispensable for maintaining physical growth in this boy. Thus, it appears that circulating GH and IGF-1 are not mandatory requirements for maintaining normal physical growth, and other, as yet uncharacterized, pathways or growth factors might be sufficiently compensatory under certain conditions.