White-throated sparrows (Zonotrichia albicollis) exhibit a genetic polymorphism that affects plumage and behavior in both sexes. White-striped morphs are more territorially aggressive, whereas tan-striped morphs provision nestlings at a higher rate. We investigated testosterone physiology in this species in an effort to understand hormonal mechanisms for the observed differences in aggression a...

BACKGROUND Testosterone levels naturally decline with age in men, often resulting in testosterone deficiency (hypogonadism). However, few studies have examined hypogonadal characteristics and treatment in older (≥65 years) men. OBJECTIVE To compare data at baseline and after 12 months of testosterone replacement therapy (TRT) in hypogonadal men ≥65 vs <65 years old. Data for participants 65-7...

Lambert CP, Bopp MM, Johnson LE, Sullivan DH. Resistance Training and Testosterone Replacement Induced Changes in Body Composition, Free Testosterone, IGF-I, and IGFBP-3 in the Frail Elderly. JEPonline 2007;10(1):48-56. The purpose of this investigation was to examine the effects of two resistance exercise intensities and testosterone replacement on changes in circulating hormone/growth factor ...

Previous studies suggest that male testosterone concentrations have declined over time. To explore this in a large US population, we examined testosterone and free testosterone concentrations in National Health and Nutrition Examination Surveys (NHANES) from 1988-1991 and 1999-2004. We also examined sex hormone-binding globulin (SHBG), estradiol, and androstanediol glucuronide (3α-diol-G) over ...

This study investigates the effects of captivity and testosterone treatment on the volumes of brain regions involved in processing visual and spatial information in adult dark-eyed juncos (Junco hyemalis). We treated captive and free-living male juncos with either testosterone-filled or empty implants. Captive juncos had a smaller hippocampal formation (HF) (both in absolute volume and relative...

Testosterone increases muscle mass and strength and regulates other physiological processes, but we do not know whether testosterone effects are dose dependent and whether dose requirements for maintaining various androgen-dependent processes are similar. To determine the effects of graded doses of testosterone on body composition, muscle size, strength, power, sexual and cognitive functions, p...

OBJECTIVE There is a high prevalence of hypogonadism in men with type 2 diabetes. This will lead to an increase in assessments of hypogonadism. Statins could potentially decrease testosterone levels by reducing the availability of cholesterol for androgen synthesis. We compared testosterone levels and hypogonadal symptoms with statin use in a cross-sectional study of 355 men with type 2 diabete...

CONTEXT Anorexia nervosa and normal-weight hypothalamic amenorrhea are characterized by hypogonadism and hypercortisolemia. However, it is not known whether these endocrine abnormalities result in reductions in adrenal and/ or ovarian androgens or androgen precursors in such women, nor is it known whether relative androgen deficiency contributes to abnormalities in bone density and body composi...

BACKGROUND During testosterone (T) therapy, T is partly converted to 17beta-estradiol (E2) and 5alpha-dihydrotestosterone (DHT). Effects of age, testosterone dose, and body composition on total and free E2 and DHT levels are unknown. OBJECTIVE We evaluated age and dose-related differences in E2 and DHT levels in response to graded doses of testosterone enanthate in young and older men. METH...

Context: There is growing interest in the role of low testosterone levels on health in women. However, the effects of lower testosterone levels on decline in physical performance or muscle strength in older women have not yet been investigated. Objective: To investigate the associations between low testosterone levels and three year change in muscle strength and physical performance in a popula...