Special topics in endocrinology and metabolism最新文献

In the prepubertal child, the hypothalamic-pituitary-gonadal (H-P-G) axis is functional and extremely sensitive to negative feedback inhibition by low circulating levels of sex steroids. This feedback system may be under the control of unknown CNS inhibitory mechanisms. Clinical signs of puberty are preceded by increased pulsatile secretion of hypothalamic gonadotropin-releasing hormone (GnRH) followed by increased pituitary responsiveness to GnRH. Gonadotropin secretion, particularly LH, increases in both sexes, especially during sleep, resulting in gonadal stimulation, secretion of sex steroids, and progressive physical maturation. When any phase of the H-P-G axis malfunctions, abnormal puberty can result. Abnormal puberty may be precocious or delayed. When puberty is precocious it may be isosexual or heterosexual, complete or partial, intermittent (unsustained), or progressive. True (central) precocious puberty is usually progressive, and hormonally reflective of normal puberty, although occurring at an earlier age, whereas intermittent or unsustained precocious puberty usually is associated with immature patterns of gonadotropin secretion, or with complete gonadotropin suppression as in precocious pseudopuberty (ovarian or adrenal tumors). Cranial axial tomography, gonadotropin response to GnRH, and pelvic ultrasound in girls are useful tools to aid in the differential diagnosis of these conditions. Intermittent, or unsustained, puberty in girls is usually self-limited, requiring no medical or surgical intervention. True progressive central precocity may now be managed with GnRH analogues, which effectively arrest pubertal changes as well as slow rapid linear growth and skeletal maturation. Although a maturation lag usually explains most patterns of delayed puberty, it is often challenging to exclude other conditions that may contribute to slow pubertal progression, such as chronic illness, excessive exercise, emotional stress, anorexia, or drug use. Elevated serum gonadotropin levels direct further evaluation toward etiologies of gonadal failure, including gonadal dysgenesis, Klinefelter syndrome, and chemotherapy/irradiation damage. Both low gonadotropins and absence of or immature gonadotropin response to GnRH administration after a bone age of 11 years in girls and 13 years in boys point toward hypopituitarism or isolated hypogonadotropic hypogonadism. Management with administration of gradually incremented amounts of sex steroids at an appropriate psychologic age usually leads to enhanced linear growth, physical maturation, and improved self-esteem.

The requirement of zinc for humans was recognized in the early 1960s. The causes of zinc deficiency include malnutrition, alcoholism, malabsorption, extensive burns, chronic debilitating disorders, and chronic renal diseases; use of certain drugs such as penicillamine and, in some cases, diuretics; and genetic disorders such as acrodermatitis enteropathica and sickle cell disease. The requirement of zinc is increased in pregnancy and during growth. The clinical manifestations of severe zinc deficiency include bullous-pustular dermatitis, alopecia, diarrhea, emotional disorder, weight loss, intercurrent infections, and hypogonadism in males; zinc deficiency can be fatal if unrecognized and untreated. A moderate deficiency of zinc is characterized by growth retardation and delayed puberty in adolescents, hypogonadism in males, rough skin, poor appetite, mental lethargy, delayed wound healing, taste abnormalities, and abnormal dark adaptation. In mild cases of zinc deficiency in human subjects, we have observed oligospermia, slight weight loss, and hyperammonemia. Zinc is a growth factor. As a result of its deficiency, growth is affected adversely in many animal species and humans, probably because zinc is needed for protein and DNA synthesis and cell division. The effects of zinc and growth hormone on growth appear to be independent of each other in experimental animals. Whether zinc is required for the metabolism of somatomedin needs further investigation. Thyroid and adrenal functions do not appear to change as a result of zinc deficiency. Glucocorticoids may have an effect on zinc metabolism, although the clinical relevance of this effect is not known at present. In contrast, testicular function is affected adversely as a result of zinc deficiency in both humans and experimental animals. The effect appears to be a direct one since the hypothalamic-pituitary axis is intact, and may relate to the reduction in testicular size as a result of the need for zinc in cell division. In addition, zinc is required for the function of several testicular enzymes, although a specific role in steroidogenesis has not been identified. Zinc appears to have a role in the modulation of prolactin secretion, in the secretion and action of insulin, and in the production and biologic effects of thymic hormones. It is clear that the endocrine consequences of zinc deficiency are multiple, and that continued investigation should provide additional pathophysiologic and therapeutic insights.

Prostate cancer consists of epithelial and stromal elements that are heterogeneous with regard to androgen dependence. Nearly 80% of patients with symptomatic metastatic prostate cancer obtain prompt objective and subjective response to androgen deprivation. Surgical castration remains an effective form of therapy, has low morbidity, and obviates compliance problems with medical regimens. New LH-RH analogs offer complete medical androgen deprivation, appear as effective as estrogen therapy at 2-year follow-up, and have significantly lower cardiovascular side effects. Thus, LH-RH analogs may replace estrogen therapy for the medical management of metastatic prostate cancer. Androgen deprivation therapy has not been proved to prolong the survival of patients with prostate cancer. The optimal timing for initiation of endocrine therapy in these patients remains controversial. Techniques for predicting androgen dependence of prostate cancer are still evolving and are not yet applicable on a widespread clinical basis.

The reproductive system consists of a series of feedback loops involving the higher centers, the hypothalamus, the pituitary, and the gonads. The factors involved in physiologic restraint of the hypothalamic pituitary gonadal axis until the time of puberty are complex. The pattern (frequency and amplitude) of GnRH signal is important in regulating pituitary LH and FSH secretion. This signal can be amplified and modulated at the pituitary level at least in part by the sex steroids. Hypothalamic hypogonadism can be considered a disorder of the hypothalamic GnRH pulse generator that results in deficient or dysrhythmic GnRH release. The mechanisms underlying the abnormal GnRH release in acquired, functional disorders such as anorexia nervosa and amenorrhea of joggers remain controversial. Evaluation of patients with hypothalamic hypogonadism involves exclusion of hyperprolactinemia, space-occupying lesions, and other systemic disorders. The pulsatile administration of GnRH for induction of fertility represents a major advance in the treatment of these patients.

Hirsutism in women may be defined as excessive thick (terminal) hair growth in facial and body regions. It is one of the early manifestations of virilization that correlate closely with elevated testosterone production. Testosterone production rates in normal women average 0.2 mg/day, with 25% secreted by the ovaries, 25% by the adrenals, and 50% arising from the peripheral metabolism of prehormones, notably androstenedione. Increased testosterone from adrenal and/or ovarian sources induces 5 alpha-reductase activity within the susceptible hair follicle. This results in the local production of dihydrotestosterone, the potent androgen that is likely responsible for the growth and stimulation of the hair follicle that leads to hirsutism. Activation of the hair follicle by androgens provides a secondary pathway for testosterone metabolism, unfortunately at the expense of undesirable hair growth. Although virilization in women may be caused by exogenous androgens, it occurs primarily from diseases of the adrenals or ovaries. Androgen-producing tumors of the adrenals cause virilization in association with excessive production of a wide variety of C19 androgens. In contrast, ovarian tumors tend to secrete a narrower range of androgens and their presence may be more occult. The most common causes of hirsutism in women arise from nontumorous states, chiefly ovarian in origin. The androgenized ovary syndrome represents a spectrum of abnormalities ranging from idiopathic hirsutism to the polycystic ovary syndrome to ovarian hyperthecosis. These states are associated with mild to severe abnormalities of androgen production and concomitant mild to severe abnormalities of ovarian histology. The pathogenesis of these abnormalities is still speculative, but appears to be related to increased pulsatile and tonic secretion of LH with ovarian hyperstimulation. Of the various laboratory tests to evaluate hirsutism, simple measurements of plasma testosterone, free testosterone, and most recently androstanediol glucuronide seem to provide the best chemical evidence of androgen abnormalities. Treatment of hirsutism/virilism in women is difficult and frequently unsatisfactory. At present, treatment schemes include local methods, suppression of androgens via glucocorticoids or oral contraceptives, and antiandrogens.

There has been an explosion of knowledge in disorders of purine and pyrimidine metabolism during the last 20 years. During this time, more than 10 diseases have been discovered and their metabolic bases studied. Hyperuricemia and gout remain the most common clinical disorder. Rarely these disorders are explainable by an inherited enzyme abnormally, such as hypoxanthine-guanine phosphoribosyltransferase deficiency, phosphoribosyl-pyrophosphate synthetase deficiency, or glucose-6-phosphatase deficiency. The description of immunodeficiency syndromes in association with purine enzyme deficiency has led to a novel area of investigation encompassing the biochemical basis for immune function. Although less information is available concerning the other diseases associated with renal calculi, myopathy, anemia, and central nervous system dysfunction, further research will elucidate important metabolic relationships. These will no doubt expand our understanding of the pathogenesis of these disorders and provide innovative therapeutic approaches.