Infradian dynamics and variability of salivary testosterone in men and women

Background. The dynamics of testosterone levels exhibits several cyclic patterns with various period lengths. Circadian and circannual rhythms of testosterone are known in both genders. Among infradian rhythms only the circalunar cycle in women is widely accepted. In our previous studies we have found a circatrigintan (30 days) and a circavigintan (20 days) cycle in men. Whether cyclic patterns with higher frequencies are present in the dynamics of testosterone levels in men or in women is unknown.

Aim. To analyze the infradian dynamics of salivary testosterone in both genders for the presence of cyclic patterns.

Subjects and methods. Seventeen young and healthy women and 15 men were asked to collect saliva samples during 30 consecutive days. Testosterone levels were determined using radioimmunoassay, Analysis of Rhythmic Variance II (ANORVA II) was used for statistical analysis. Potential period lengths of 3 - 15 days were evaluated.

Results. The dynamics of salivary testosterone showed high intra-individual variability in both genders (coefficient of variation - 28% in women and 26% in men). ANORVA II analysis showed no significant rhythms, although a weak circaseptan cyclic pattern has been found in women.

Discussion. Our results showed no significant infradian cyclic variation with a period between 3 and 15 days. Further studies should concentrate on potential longer periods. Described intra-individual variability of testosterone levels in both genders should be considered in endocrine research.     

Influence of temperature on the infradian growth rhythm in Ulva lactuca (Chlorophyta)

The vegetative growth of Ulva lactuca was studied to determine if the growth rate of the alga is driven by infradian rhythmicity. The influence of temperature on the infradian rhythm of growth was also investigated. Discs of Ulva were grown in controlled laboratory conditions at different combinations of temperature (5, 10, 15, 20°С) and irradiance (40 and 60 μmol photons m^?2 s^?1) under 12 : 12 h light : dark cycles. The growth rates exhibited a rhythmic pattern with one major peak every 2 or 3 days. Growth at 5 or 10°С increased the prevalence of 3-day cycles and maintained U. lactuca in the vegetative growth stage. In contrast, growth at 15 or 20°С provoked a predominance of the 2-day cycle and induced reproduction. The 2- or 3-day cycles were combined in longer cycles having a period close to 6 days. We suppose that the 2-, 3- and 6-day rhythms of physiological processes are related to large-scale Rossby and Kelvin waves, which produce oscillations in the geomagnetic field and seawater temperature with the same periods. The predominance of 2-day or 3-day fluctuations of the geomagnetic field and temperature probably determine the prevalence of reproduction and vegetative growth, respectively, in Ulva.     

Lack of evidence for meteorological effects on infradian dynamics of testosterone

Climatic factors are known to influence the endocrine system. Previous studies have shown that circannual seasonal variations of testosterone might be partly explained by changes in air temperature. Whether infradian variations are affected by meteorological factors is unknown. To analyze possible effects of meteorological parameters on infradian variations of salivary testosterone levels in both sexes, daily salivary testosterone levels were measured during 1 month in 14 men and 17 women. A correlation analysis between hormonal levels and selected meteorological parameters was performed. The results indicate that high testosterone levels are loosely associated with cold, sunny and dry weather in both sexes. However, only the correlations between testosterone and air temperature (men) and actual cloudiness (women) were statistically significant (p < 0,05). Although some correlations reached the level of statistical significance, the effects of selected meteorological parameters on salivary testosterone levels remain unclear. Further longer-term studies concentrating on air temperature, cloudiness and average relative humidity in relation to the sex hormone axis are needed.     

Infradian rhythmics of markers of physiologic and metabolic processes in Wistar male rats

This work presents the results of the amplitude-phase structure of the physiological rhythms (locomotor activity) and steroid hormones production (common testosterone and cortisole) in gelded and intact Wistar male rats as well as rats exposed to an information load. The markers have been shown to depend on the social state of the animal. It was proved that there was a phase coincidence of the periods of the biological rhythms of common testosterone (intact rats), cortisole and locomotor activity for animals with different individualand typological features. It was shown that the oscillation period of infradian rhythms of the measured markers of physiologic and metabolic processes in the animals of all groups amounted to 4 days. It was found that infradian rhythms were not sensitive to factors of various genesis.     

Oscillations in joint synchrony of reproductive hormones in healthy men

Negative-feedback (inhibitory) and positive-feedforward (stimulatory) processes regulate physiological systems. Whether such processes are themselves rhythmic is not known. Here, we apply cross-approximate entropy (cross-ApEn), a noninvasive measurement of joint (pairwise) signal synchrony, to inferentially assess hypothesized circadian and ultradian variations in feedback coupling. The data comprised simultaneous measurements of three pituitary and one peripheral hormone (LH, FSH, prolactin, and testosterone) in 12 healthy men each sampled every 10 min for 4 days (5,760 min). Ergodicity, due to the time series stationarity of the measurements over the 4 days, allows for effective estimation of parameters based upon the 12 subjects. Cross-ApEn changes were quantified via moving-window estimates applied to 4-day time series pairs. The resultant ordered windowed cross-ApEn series (in time) were subjected to power spectrum analysis. Rhythmicity was assessed against the null hypothesis of randomness using 1,000 simulated periodograms derived by shuffling the interpulse-interval hormone-concentration segments and redoing cross-ApEn windows and spectral analysis. By forward cross-ApEn analysis, paired LH-testosterone, LH-prolactin, and LH-FSH synchrony maintained dominant rhythms with periodicities of 18-22.5, 18, and 22.5 h, respectively (each P < 0.001). By reverse (feedback) cross-ApEn analysis, testosterone-LH, testosterone-prolactin, and testosterone-FSH synchrony cycles were 30, 18, and 30-45 h, respectively (each P ≤ 0.001). Significant 8- or 24-h rhythms were also detected in most linkages, and maximal bihormonal synchrony occurred consistently at ～0400-0500. Collectively, these analyses demonstrate significant ultradian (<24 h), circadian (～24 h), and infradian (>24 h) oscillations in pituitary-testis synchrony, wherein maximal biglandular coordination is strongly constrained to the early morning hours.     

External factors controlling annual testosterone and thyroxine cycles in the edible dormouse Glis glis

Annual patterns of hibernation, body weight, plasma testosterone and thyroxine were measured in two groups of edible dormice exposed to a constant photoperiod and to either environmental temperature (I), the four functions displayed annual cycles and the two hormonal cycles were parallel and restricted to the aroused state, while in group II only irregular infradian fluctuations were observed. However, a body temperature of 30 degrees C remained the critical lower limit enabling the start of the testosterone cycles for the two groups. Respective roles of endogenous and synchronizing mechanisms in controlling biological rhythms are discussed.     

Entraining signals initiate behavioral circadian rhythmicity in larval zebrafish

The authors show that a circadian clock that regulates locomotor activity in larval zebrafish develops gradually over the first 4 days of life and that exposure to entraining signals late in embryonic development is necessary for initiation of robust behavioral rhythmicity. When zebrafish larvae were transferred from a light-dark (LD) cycle to constant darkness (DD) on the third or fourth day postfertilization, the locomotor activity of almost all fish was rhythmic on days 5 to 9 postfertilization, with peak activity occurring during the subjective day. Rhythm amplitude was higher after four LD cycles than after three LD cycles. When embryos were transferred from LD to DD on the second day postfertilization, only about half of the animals later displayed statistically significant activity rhythms. These rhythms were noisier and of lower amplitude, but phased normally. When zebrafish were raised in DD beginning at 14 h postfertilization, only 22% of them expressed significant circadian rhythmicity as larvae. These rhythms were of low amplitude and phase-locked to the time of handling on the third day rather than to the maternal LD cycle. These results show that behavioral rhythmicity in zebrafish is regulated by a pacemaking system that is sensitive to light by the second day of embryogenesis but continues to develop into the fourth day. This pacemaking system requires environmental signals to initiate or synchronize circadian rhythmicity.     

Diurnal variations of plasma testosterone in men

Plasma testosterone (T) levels were assayed by a Competitive Protein Binding (CPB) technique in a group of 31 healthy males. In 22 subjects a single blood sample was taken between 8:00 and 9:00 A.M. and the mean T concentration was 6.84 ± 2.11 ng/ml. In the other 9 normal men, blood samples were taken every 4 hours. The existence of temporal variations for testosterone was confirmed by finding the highest mean plasma levels at 4:00 A.M. (9.28 ± 1.17 ng/ml) and lowest mean levels at 8:00 P.M. (2.66 ± 0.52 ng/ml).     

Natural environmental associations in a 50-day human electrocardiogram

Meteorotropic associations of heart rate (HR) and HR variability (HRV) were investigated in a clinically healthy 48-year-old man in Kiev. His electrocardiogram (ECG) was determined over 50 days by fitting him with an ambulatorily wearable device; various natural physical environmental variables were also monitored. The mean inter-beat interval, the standard deviation of these intervals, the spectral power in several frequency ranges, the power ratio of the approx. 10.5-s/approx. 3.6-s spectral components and other aspects of HRV were computed over consecutive 14.4-min intervals. Together with ordinary meteorological variables, geomagnetic disturbances (GMD) and fluctuations of atmospheric pressure (FAP) in the range 0.01–0.10 Hz (10–100 s) were measured. The assessable infradian spectra (with frequencies lower than 1 cycle/28 hours) of all HRV parameters showed two major components with periods of about 3.5 and 10 days. Two environmental variables, FAP and wind speed (with which FAP is closely related), revealed both of these rhythms and showed the greatest cross-spectral coherence (0.70–0.98) with corresponding oscillations of HRV. Less specific but statistically significant product-moment correlations with major HRV indices were also found; most of these were with FAP, but correlations with air temperature, humidity, wind speed and geomagnetic disturbances were also found. Long-term ECG recording, essential in the detection of infradian rhythms, proved to be sensitive to physical environmental variables, notably meteorological ones. FAP, usually neglected since its role has not been considered in previous biometeorological studies, or some factor closely related to FAP is probably involved in synchronizing or influencing the approximately 3.5-day HR and HRV rhythms in humans. Received: 27 July 2000 / Revised: 13 November 2000 / Accepted: 28 November 2000     

Effect of Time‐of‐Day‐Specific Strength Training on Serum Hormone Concentrations and Isometric Strength in Men

A time‐of‐day influence on the neuromuscular response to strength training has been previously reported. However, no scientific study has examined the influence of the time of day when strength training is performed on hormonal adaptations. Therefore, the primary purpose of this study was to examine the effects of time‐of‐day‐specific strength training on resting serum concentrations and diurnal patterns of testosterone (T) and cortisol (CORT) as well as maximum isometric strength of knee extensors. Thirty eight diurnally active healthy, previously untrained men (age 20–45 yrs) underwent a ten‐week preparatory strength training period when sessions were conducted between 17:00–19:00 h. Thereafter, these subjects were randomized into either a morning (n=20, training times 07:00–09:00 h) or afternoon (n=18, 7:00–19:00 h) training group for another ten‐week period of time‐of‐day‐specific training (TST). Isometric unilateral knee extension peak torque (MVC) was measured at 07:00, 12:00, 17:00, and 20:30 h over two consecutive days (Day 1 & Day 2) before and after TST. Blood samples were obtained before each clock‐time measurement to assess resting serum T and CORT concentrations. A matched control group (n=11) did not train but participated in the tests. Serum T and CORT concentrations significantly declined from 07:00 to 20:30 h on all test days (Time effect, p<.001). Serum CORT at 07:00 h was significantly higher on Day 1 than Day 2 in the control and afternoon group, both in Pre and Post conditions (Day×Time interaction, p<.01). In the morning group, a similar day‐to‐day difference was present in the Pre but not Post conditions (Time×Group interaction, p<.05). MVC significantly increased after TST in both the morning and afternoon groups (Pre to Post effect, p<.001). In both groups, a typical diurnal variation in MVC (Time effect, p<.001) was found, especially on Day 2 in the Pre condition, and this feature persisted from Pre to Post in the afternoon group. In the morning group, however, diurnal variation was reduced after TST on both Day 1 and Day 2 (Pre to Post×Day×Time×Group interaction, p<.05). In conclusion, 10 weeks of morning time‐of‐day‐specific strength training resulted in reduced morning resting CORT concentrations, presumably as a result of decreased masking effects of anticipatory psychological stress prior to the morning testing. The typical diurnal pattern of maximum isometric strength was blunted by the TST period in the morning but not the afternoon group. However, the TST period had no significant effect on the resting total T concentration and its diurnal pattern and on the absolute increase in maximum strength.     

Physiology and regulation of biological rhythms in laboratory animals: an overview

Biological rhythms have been observed in practically all groups of laboratory mammals and at every level of physiological and behavioural organization. Biological rhythms are classified according to their period as ultradian (less than 24 h), circadian (approximately 24 h), infradian (greater than 24 h), and seasonal or circannual rhythms (approximately 1 year). This review outlines what is known about the neurobiology of biological rhythms in mammals and describes the hierarchical order in which ultradian, circadian and infradian rhythms are related to each other. The article does not attempt to catalogue every physiological variable showing rhythmical fluctuations in laboratory mammals. Rather, it focuses on the basic concepts of circadian rhythms and recent advances made in our understanding of the physiology of the internal clock controlling circadian and other biological rhythms.     

The effect of temperature on infradian rhythms of reproduction in <Emphasis Type="Italic">Ulva fenestrata</Emphasis> postels et ruprecht, 1840 (Chlorophyta: <Emphasis Type="Italic">Ulvales</Emphasis>)

The influence of temperature on the duration of infradian (over 28 h) rhythms of reproduction in the green alga Ulva fenestrata was studied in the laboratory under controlled conditions combining four temperatures (5, 10, 15, and 20°C), two irradiance levels (40 and 60 μE/(m² s)) and neutral daylengths (12: 12 h light: dark). The rise in water temperature explained 78.4% of the reduction in the reproduction cycles (from 30 to 5 days). It is suggested that U. fenestrata has an endogenous reproductive rhythm with a period of 5 days. In unfavorable conditions, one or more reproduction cycles are omitted, and formation of gametes or spores attains a periodicity of 10, 15, or more days.     

Supradian and infradian cycles in oxygen uptake of diapausing pupae of Pieris brassicae

The general characteristics of diapause respiration in P. brassicae are described, together with an examination of short-term (supradian) and long-term (infradian) variation in oxygen uptake. Supradian cycles occur approximately every 3 hr at 10°C and are shown by closed box analyses to be initiated by carbon dioxide bursts. Maximal rates of oxygen uptake occur shortly after the burst in carbon dioxide release, not at the start of the burst as recorded in other diapausing species. The frequency of supradian cycles is directly related to temperature and metabolism in accordance with the characteristics of discontinuous carbon dioxide release.Infradian cycles of between 3 and 7 days duration are recorded for both oxygen uptake and net exchange rates. Peaks in oxygen demand occur on average every 4 days at 10°C, and are related in frequency to the level of metabolism of individual pupae. Just before post-diapause development, oxygen demands fall to about half their normal levels; these changes are associated with appropriate changes in the frequency of supradian and infradian cycles.     

A temperature-dependent timing mechanism is involved in the circadian system that drives locomotor rhythms in the fruit fly Drosophila melanogaster

The circadian clock of Drosophila melanogaster is thought to include rhythmic expression of period gene. Recent studies suggested, however, that a per-less oscillation is also involved in the regulation of circadian locomotor rhythms. In the present study, we examined the existence and the property of the possible per-less oscillation using arrhythmic clock mutant flies carrying per (01), tim(01), dClk(Jrk) or cyc(01), which lack rhythmic per expression. When temperature cycles consisting of 25 degrees C and 30 degrees C with various periods (T=8-32 hr) were given, wild-type (Canton-S) flies showed locomotor rhythms entrained to temperature cycles over a wide range of period (T=8-32 hr) in constant light (LL) while only to T=24 hr in constant darkness (DD). The mutant flies showed rhythms synchronizing with the given cycle both under LL and DD. In per(01) and tim(01) flies, the phase of a major peak slightly changed dependent on Ts in DD, while it did not in dClk(Jrk) and cyc(01) flies. When they were transferred from a constant temperature to a temperature cycle under DD, several cycles were necessary to establish a clear temperature entrainment in per(01) and tim (01) flies. These results suggest that per(01) and tim(01) flies have a temperature-entrainable weak oscillatory mechanism and that the per-less oscillatory mechanism may require dClk and cyc. In addition, per (01) and tim(01) flies changed from thermoactive in DD to cryoactive in LL, while dClk(Jrk) and cyc(01) flies did not. It is thus suggested that dClk and cyc are also involved in determining the light-associated temperature preference in per(01) and tim(01) flies.     

Daily Rhythms in Blood and Milk Lead Toxicokinetics Following Intravenous Administration of Lead Acetate to Dairy Cows in Winter

The objective was to investigate circadian variations of blood and milk lead toxicokinetics in dairy cows in winter. Twenty lactating Holstein animals were randomly assigned to 4 treatments, corresponding to different hours after onset of light (HALO): 2, 8, 14 and 20. Cows received a single intravenous administration of 2.5 mg/kg lead as lead acetate. Blood and milk samples were taken and analyzed by atomic absorption spectrophotometry. For each toxicokinetic parameter, a one-way analysis of variance was performed to outline the existence of daily variations. Significant differences as a function of HALO were detected in blood for the hybrid constant of elimination (β), half-life of elimination (t_1/2β), area under the curve (AUC) and clearance (Cl) (p &lt; 0.01) and volume of distribution at steady state (V_ss) (p < 0.05). Half-life of elimination was highest when lead acetatae was injected at 2 HALO, and lowest following the 14 HALO administration. Milk data showed significant differences for maximum concentration, AUC and Cl (p &lt; 0.001), volume of distribution and V_ss (p < 0.005). The ratio AUC_milk/ABC_blood was utilized to estimate penetration of lead in milk. It differed significantly along the day (p < 0.01). Blood daily variations could be fitted a circadian rhythms. No circadian rhythms were detected in milk parameters or in the ratio AUC_milk/ABC_blood. It was concluded that blood and milk lead toxicokinetics are distinctly affected by the time of the day.     

The daily,weekly, and seasonal cycles of body temperature analyzed at large scale

ABSTRACT

We performed large-scale analyses of circadian and infradian cycles of human body temperature, focusing on changes over the day, week, and year. Temperatures (n= 93,225) were collected using temporal artery thermometers from a Boston emergency department during 2009–2012 and were statistically analyzed using regression with cyclic splines. The overall mean body temperature was 36.7°C (98.1°F), with a 95% confidence interval of 36.7–36.7°C (98.1–98.1°F) and a standard deviation of 0.6°C (1.1°F). Over the day, mean body temperature followed a steady cycle, reaching its minimum at 6:00–8:00 and its maximum at 18:00–20:00. Across days of the week, this diurnal cycle was essentially unchanged, even though activities and sleeping hours change substantially during the weekly cycles of human behavior. Over the year, body temperatures were slightly colder in winter than summer (~0.2°C difference), consistent with most prior studies. We propose these seasonal differences might be due to ambient effects on body temperature that are not eliminated because they fall within the tolerance range of the thermoregulatory system. Over the year, bathyphase (daily time of minimum temperature) appeared to parallel sunrise times, as expected from sunrise’s zeitgeber role in circadian rhythms. However, orthophase (daily time of maximum temperature) and sunset times followed opposite seasonal patterns, with orthophase preceding nightfall in summer and following nightfall in winter. Throughout the year, bathyphase and orthophase remained separated by approximately 12 h, suggesting this interval might be conserved. Finally, although 37.0°C (98.6°F) is widely recognized as the mean or normal human body temperature, analysis showed mean temperature was <37.0°C during all times of day, days of the week, and seasons of the year, supporting prior arguments that the 37.0°C standard has no scientific basis. Overall, this large study showed robust and consistent behavior of the human circadian cycle at the population level, providing a strong example of circadian homeostasis.     

Changes in the diurnal rhythms of cortisol,melatonin, and testosterone after 2, 4, and 7 consecutive night shifts in male police officers

Night work is associated with a large range of acute health problems and possibly also health consequences in the long run. Yet, only very few field studies specifically investigate the effects of consecutive night shift on key physiological regulatory systems. In this field study, we investigated the effects of consecutive night shifts on three hormones, melatonin, cortisol, and testosterone, among police officers at work. More specifically, the aim was to investigate how the diurnal rhythms of melatonin, cortisol, and testosterone responded to two, four, and seven consecutive night shifts and a corresponding number of days for recovery. The study was part of the “In the Middle of the Night” project and included 73 male police officers from five different police districts. The participants were exposed to three intervention conditions: “2+2”: two consecutive night shifts followed by two consecutive day recovery days; “4+4”: four consecutive night shifts followed by four consecutive recovery days; “7+7”: seven consecutive night shifts followed by seven consecutive recovery days. On the last day with night shift and the last recovery day in each intervention, the participants collected saliva samples every 4th hour when awake. The diurnal rhythms of melatonin, cortisol, and testosterone were all affected differently by an increasing number of consecutive night shifts: the amplitude of the melatonin rhythm was suppressed by 4.9% per day (95% CI 1.4–8.2% per day; p = 0.006). The diurnal rhythm of cortisol phase was delayed with an increasing number of night shifts by 33 min/day (95% CI 18–48 min per day; p ≤ 0.001), but did not show any changes in amplitude. For the diurnal rhythm of testosterone, there was no effect of the number of consecutive night shifts and the diurnal rhythm completely followed the sleep/wake cycle. We found that there were no differences in the rhythms of melatonin, cortisol, and testosterone after 2, 4, and 7 recovery days, respectively. In conclusion, we found signs of desynchronization in terms of suppressed amplitude of melatonin and phase delay of salivary cortisol as a consequence of the increasing number of consecutive night shifts among police officers at work. Lack of synchronization has been suggested as a possible mechanism linking night work to disease, but this remains to be determined.     

Circadian components of semilunar and lunar timing mechanisms

The timing of semilunar as well as lunar reproductive rhythms has been analyzed in different geographic populations of the intertidal chironomid Clunio. In stocks of three populations differing in period and phase relationship with the lunar month, these long-term rhythms were synchronized in the laboratory by using artificial moonlight cycles of 30 days in otherwise 24-hr light-dark (LD) cycles (0.4 lux during 4 successive nights every 30 days in LD 12:12). In LD cycles of various periods, a strong synchronization was only possible in LD 12:12 and LD 11:11, whereas in LD 10:10 and LD 15:15 the synchronization by the 30-"day" moonlight cycle was weak or even absent. The study demonstrates a limited range of circadian periods for entrainment of the long-term rhythms. It is concluded that an LD cycle with a period near 24 hr is an essential zeitgeber condition for semilunar and lunar timing in this marine insect. Further, it is suggested that the underlying physiological timing mechanism of Clunio consists of a circadian function for the perception of the monthly moonlight zeitgeber cycles that entrain the endogenous, temperature-compensated oscillator of the circasemilunar (or circalunar) period. The long-term oscillator triggers the metamorphosis of the insect, and thereby determines the time of its eclosion and reproduction on the shorelines, in correlation with days of spring tides recurring about every 14-15 days.     

Testosterone Levels in Athletes at Rest and Exhaustion: Effects of Calcium Supplementation

The effects of 4 weeks of calcium supplementation on free- and total testosterone levels were established in active and sedentary adult males at rest and exhaustion. Thirty healthy male athletes were equally divided into three study groups, as follows: Group 1—non-exercising subjects receiving 35 mg calcium/kg body weight; Group 2—subjects receiving 35 mg calcium/kg body weight undergoing training routines for 90 min/day, 5 days a week and Group 3—subjects undergoing training routines for 90 min/day, 5 days a week. The testosterone levels were determined before and after supplementation, at rest and following a hard training routine. The plasma free- and total testosterone levels increased at exhaustion before and after supplementation relative to resting values (p?<?0.05). This was also true when active subjects were compared to inactive subjects (p?<?0.05). Our results show that training results in increased testosterone levels in athletes and that the increase is greater if accompanied by calcium supplementation, which may be useful for increasing overall athletic performance.     

Valoración de niveles de testosterona mediante diferentes métodos analíticos en varones sanos

Plasma testosterone concentrations are essential for the diagnosis of several causes of hypogonadism, including late-onset hypogonadism. Defining the normal range for testosterone concentrations poses certain difficulties due to the changes that occur with age and the variability of the different analytical methods used.ObjectivesTo study normal ranges of testosterone in healthy young men and to compare the results of distinct analytical methods.Material and methodsWe recruited 20 healthy men with a mean age of 24.5 years (standard deviation (SD): 5.04) and a mean body mass index (BMI) of 23.8% (SD: 3.3). Total testosterone (TT) was measured by immunochemiluminescence (ICLA) and free testosterone (FT) by radioimmunoassay (RIA). Calculated free testosterone (FTc) and bioavailable testosterone (BT) were calculated using Vermeulen's formula. Serum lutropin (LH), follitropin (FSH) and sex hormone binding globulin (SHBG) were measured by immunoradiometric assays (IRMA).ResultsThe mean concentrations were 20 nmol/l (SD: 4.96) for TT, 0.054 nmol/L (SD: 0.01) for FT, 0.3834 nmol/L (SD: 0.09) for FTc and 9.9 nmol/L (SD: 2.8) for BT. There was no correlation between testosterone measured by different methods other than an association between FT and FTc (r=0.662, p<0.003) and between FTc and BT (r=0.979, p<0.0001). An inverse correlation was found between BMI and TT concentrations (r: ?0.52, p<0.017).ConclusionsThe normal range for testosterone in healthy young men should be established in each laboratory based on the analytical method used.