Time-Related Behaviour of Endocrine Secretion: Circannual Variations of FT3, Cortisol,Hgh and Serum Basal Insulin in Healthy Subjects

Four healthy young male volunteers were submitted to the study of circadian and circannual bioperiodicities of several hormones: FT3, FT4, Cortisol, HGH, prolactin, PTh and plasma insulin levels. They were observed for a whole year and their blood samples were collected six times a day, every other month. The results were analyzed by two-way ANOVA macroscopic analysis and Student r-test. Our data registered a circannual variation in the mean circadian plasma levels of the following hormones: Cortisol (peak in December), HGH (peak in April), FT3 (peak in April), insulin (peak in February). FT4, prolactin and PTH showed no cyclic variation during the period of observation.     

Time-Related Behaviour of Endocrine Secretion: Circannual Variations of FT3, Cortisol, Hgh and Serum Basal Insulin in Healthy Subjects

Four healthy young male volunteers were submitted to the study of circadian and circannual bioperiodicities of several hormones: FT3, FT4, Cortisol, HGH, prolactin, PTh and plasma insulin levels. They were observed for a whole year and their blood samples were collected six times a day, every other month. The results were analyzed by two-way ANOVA macroscopic analysis and Student r-test. Our data registered a circannual variation in the mean circadian plasma levels of the following hormones: Cortisol (peak in December), HGH (peak in April), FT3 (peak in April), insulin (peak in February). FT4, prolactin and PTH showed no cyclic variation during the period of observation.     

Circadian and circannual rhythms of LH, FSH, testosterone (T), prolactin, cortisol, T3 and T4 in plasma of mature, male white-tailed deer

Circadian and circannual rhythm of plasma LH, FSH, testosterone (T), prolactin, cortisol, triiodothyronine (T3) and thyroxine (T4) were investigated in two mature male white-tailed deer. No circadian rhythms were detected. Seasonal levels of LH and FSH were reached in September and October; troughs occur in May and June. Maximal T values were detected in November and December (the time of the rut); minimal levels occur between February and July. Prolactin peaked in May and June; minimal levels were detected between October and February. T3 exhibited two maxima; the first in the May-June period, the second in the September-October period. T4 showed no recognizable circannual rhythm. Cortisol levels were found to be much higher during cold months (December-April) than during the rest of the year. The least variable circadian levels were that of FSH and prolactin, with LH, T4, T3, cortisol and testosterone following in descending order. Cannulation stress might have some effect on the levels of testosterone, LH and cortisol. Correlation between LH and testosterone levels were detected mainly during sexually active periods.     

Neuro-endocrine correlations of hypothalamic-pituitary-thyroid axis in healthy humans

Neuro-endocrine hormone secretion is characterized by circadian rhythmicity. Melatonin, GRH and GH are secreted during the night, CRH and ACTH secretion peak in the morning, determining the circadian rhythm of cortisol secretion, TRH and TSH show circadian variations with higher levels at night. Thyroxine levels do not change with clear circadian rhythmicity. In this paper we have considered a possible influence of cortisol and melatonin on hypothalamic-pituitary-thyroid axis function in humans. Melatonin, cortisol, TRH, TSH and FT4 serum levels were determined in blood samples obtained every four hours for 24 hours from ten healthy males, aged 36-51 years. We correlated hormone serum levels at each sampling time and evaluated the presence of circadian rhythmicity of hormone secretion. In the activity phase (06:00 h-10:00 h-14:00 h) cortisol correlated negatively with FT4, TSH correlated positively with TRH, TRH correlated positively with FT4 and melatonin correlated positively with TSH. In the resting phase (18:00 h-22:00 h-02:00 h) TRH correlated positively with FT4, melatonin correlated negatively with FT4, TSH correlated negatively with FT4, cortisol correlated positively with FT4 and TSH correlated positively with TRH. A clear circadian rhythm was validated for the time-qualified changes of melatonin and TSH secretion (with acrophase during the night), for cortisol serum levels (with acrophase in the morning), but not for TRH and FT4 serum level changes. In conclusion, the hypothalamic-pituitary-thyroid axis function may be modulated by cortisol and melatonin serum levels and by their circadian rhythmicity of variation.     

A method to evaluate dynamics and periodicity of hormone secretion

Spontaneous hormone secretory dynamics include tonic and pulsatile components and a number of periodic processes. Circadian variations are usually found for melatonin, TSH and GH, with peak secretions at night, and in cortisol secretion, which peaks in the morning. Free thyroxine (FT4) and insulin-like growth factor (IGF)1 levels do not always change with circadian rhythmicity or show only minor fluctuations. Fractional variations explore the dynamics of secretion related to time intervals, and the rate of change in serum levels represents a signal for the receptorial system and the target organ. We evaluated time-related variations and change dynamics for melatonin, cortisol, TSH, FT4, GH and IGF1 levels in blood samples obtained every 4 h for 24 h from eleven healthy males, ages 35-53 years (mean ? SE 43.6 ± 1.7). Nyctohemeral (i.e., day-night) patterns of hormone secretion levels and the fractional rate of variation between consecutive 4-hourly time-qualified hormone serum levels (calculated as percent change from time 1 to time 2) were evaluated for circadian periodicity using a 24 and 12-h cosine model. A circadian rhythm was validated for serum level changes in cortisol with peaks of the 24-h cosine model at 07:48 h, and melatonin, TSH and GH, with phases at 01:35 h, 23:32 h, and 00:00 h, respectively. A weak, but significant, 12-h periodicity was found for FT4 serum levels, with minor peaks in the morning (10:00 h) and evening (22:00 h), and for IGF1, with minor peaks in the morning (07:40 h) and evening (19:40 h). Circadian rhythmicity was found in the 4-hourly fractional variations with phases of increase or surge at 02:00 h for cortisol, 22:29 h for melatonin, 05:14 h for FT4, and 21:19 h for GH. A significant 12-h periodicity was found for the 4-hourly fractional variations of TSH with two peaks in the morning (decrease or drop at 04:42 h) and afternoon (surge at 16:28 h), whereas IGF1 fractional variation changes did not show a significant rhythmic pattern. In conclusion, the calculation of the time-qualified fractional rate of variation allows evaluation of the dynamics of secretion and the specification of the timepoint(s) of maximal change of secretion, not only for hormones whose secretion is characterized by a circadian pattern of variation, but also for hormones that show no circadian or only weak ultradian (12 h) variations (i.e., FT4).     

Circannual rhythm of insulin release and hypoglycemic effect of tolbutamide stimulus in healthy man

Four healthy non obese young volunteers were observed for a 24-hr period, every other month, over the course of one year. Tolbutamide was injected i.v. each day of the experiment every four hours. Tolbutamide-induced insulin secretion (T.I.I.S.) was evaluated by planimetrically measuring insulin areas above basal levels. Tolbutamide-induced hypoglycemic effect was evaluated by measuring the blood glucose difference between the 5th and 25th minute after the drug injection (delta G5'-25'). The macroscopic evaluation of T.I.I.S. and delta G5'-25' (mean chronograms) permitted the detection of the existence of a circannual variation of both variables. In particular the maximum level of the blood glucose drop (delta G5'-25') was registered in February. Subsequently the quantification of the rhythm of T.I.I.S. was obtained by fitting a sine curve, according to the Cosinor method. The highest insulin release was confirmed in winter. As previously documented, the existence of a statistically significant circadian rhythm of T.I.I.S. was confirmed in the morning, i.e. the same period of the day in which insulin-induced hypoglycemia occurs.     

Circannual Rhythm of Insulin Release and Hypoglycemic Effect of Tolbutamide Stimulus in Healthy Man

Four healthy non obese young volunteers were observed for a 24-hr period, every other month, over the course of one year. Tolbutamide was injected i.v. each day of the experiment every four hours. Tolbutamide-induced insulin secretion (T.I.I.S.) was evaluated by planimetrically measuring insulin areas above basal levels. Tolbutamide-induced hypoglycemic effect was evaluated by measuring the blood glucose difference between the Sth and 25th minute after the drug injection (δG 5′-25′). The macroscopic evaluation of T.I.I.S. and δG 5′-2S′(mean chronograms) permitted the detection of the existence of a circannual variation of both variables. In particular the maximum level of the blood glucose drop (δG 5-25) was registered in February.

Subsequently the quantification of the rhythm of T.I.I.S. was obtained by fitting a sine curve, according to the Cosinor method. The highest insulin release was confirmed in winter.

As previously documented, the existence of a statistically significant circadian rhythm of T.I.I.S. was confirmed in the morning, i.e. the same period of the day in which insulin-induced hypoglycemia occurs.     

Circannual versus seasonal variations of longitudinally sampled 25-hydroxycholecalciferol serum levels

Seasonal variations in human serum levels of 25-hydroxycholecalciferol (25OHD) have been largely documented in transverse studies of population. But seasonality is not per se a demonstration that 25OHD serum levels fluctuate along the course of year according to a waveform profile with a periodic rhythm. Because of this, we attempted to investigate the possible occurrence of a circannual rhythm for 25OHD serum levels in a longitudinal design, by fitting a 365.25-day cosine curve to temporal biodata recorded in 10 clinically healthy subjects, monthly sampled for RIA determinations of 25OHD. Cosinor procedure statistically validated the occurrence of a circannual rhythm for 25OHD serum concentrations at a highly significant level of probability (P = 0.0015) for null hypothesis amplitude = 0. With 95% of probability, amplitude ranges from 5.0 to 16.5 ng/ml (mean value of oscillation = 10 ng/ml), while acrophase is temporally located from September 14 to December 3 (mean timing = October 21). Yearly, mean values for 25OHD serum concentrations is of 40.3 +/- 5.4 ng/ml as quantified by the line which transversely divides the cosine curve interpolating original biodata. By calculating the band of a complete 12 months variability which includes 90% of the distribution with 90% confidence limits, the circannual chronodesm of 25OHD serum levels has been obtained. Such a chronodesmic sinusoid has been compared to the circannual chronogram. By this comparison, a dissociation between the crest (October) and the peak (August) has been detected. The finding suggests that seasonal variations are superimposed to the circannual rhythm. Seasonal but also circannual changes, thus, characterize the yearly variability of 25OHD serum levels in man.     

Mode of action of somatostatin in inhibiting parathyroid hormone secretion

Effects of somatostatin on basal and low calcium-, isoproterenol- or dibutryl cyclic AMP (DBcAMP)-stimulated parathyroid hormone (PTH) secretion were evaluated in vitro with bovine parathyroid tissue. Low calcium, isoproterenol or DBcAMP alone significantly stimulated PTH secretion. Somatostatin 1 or 4 microgram/ml significantly inhibited these stimulated PTH secretions. Inhibition of isoproterenol-stimulated PTH secretion was more complete than was the inhibition of low calcium- or DBcAMP-stimulated secretion. The studies indicate that somatostatin inhibits PTH secretion by an action distal to cAMP generation. The more complete inhibition of isoproterenol-stimulated PTH secretion suggests that somatostatin may also have additional effects on or proximal to the formation of cyclic AMP.     

Prolactin, cortisol secretion and thyroid function in patients with stroke of mild severity.

Different attempts were made to identify the variables that may be involved in the clinical course of cerebrovascular ischemia. In the case of stroke with mild severity (SMS), the clinical significance of neuroendocrine changes as well as of post-stroke depression (PSD) remains unknown. We therefore evaluated the presence of neuroendocrine changes in the acute and post-acute phase of SMS, and their potential role during convalescence. Serum cortisol, T4, T3, FT4, FT3, TSH and PRL levels were measured in 17 euthyroid patients with stroke on admission (day 1), following morning (day 2), 7 days and 3 months later. TSH and PRL secretion after TRH test were measured. Stroke severity on admission was determined by Scandinavian Stroke Scale (SSS). Montgomery-Asberg Depression Rating Scale (Madrs) was used for assessment of post-stroke depression. On admission, TSH and T3, were within normal limits and were greater compared to values on day 2. Lower basal TSH and decreased TSH response to TRH on day 2, were associated with stroke of greater severity. Delta-PRL after TRH on day 2 was higher in patients who develop PSD. Changes in serum thyroid hormones in SMS, reflects those of non-thyroidal illness. A mild stimulation of hypothalamic-pituitary-adrenal axis was detected. We provide evidence that PRL response to TRH, in the acute phase of stroke may be used as an index for early detection of PSD.     

Circannual fluctuations of the serum cortisol in the European ground squirrel, Citellus citellus L

1. Serum cortisol levels were measured throughout an annual cycle in male European ground squirrels, Citellus citellus L. 2. A circannual rhythm of these levels in euthermic animals was found: the highest being in late October, November and December, the lowest in May, early October and March. 3. The levels varied in the hibernating period as well: strongly reduced during the first part (October-December) and significantly elevated in the second one (January-March). 4. A significant elevation of the cortisol levels was marked 18-20 hr after full arousal from hibernation in October.     

The influence of plasma triglycerides on human growth hormone response to arginine and insulin: a study in hyperlipemics and normal subjects.

Human growth hormone (HGH) response to arginine (25 gm IV in 30 min) and to insulin (0.1 U/kg B.W.) was studied in 12 male patients (mean age 36 +/- 2 years), with normal glucose tolerance and normal body weight, affected with Fredrickson's Type IV primary hyperlipemia. The patients were examined both when plasma triglycerides (TG) were elevated and following clofibrate (2 gm/die for 30-60 days) induced TG reduction. No variations in glucose or FFA behaviour or in body weight were observed after clofibrate. HGH response to arginine was absent, while that to insulin was only inhibited, when plasma TG were elevated. A significant increase in HGH peaks after arginine (from 1.99 +/- 0.59 to 9.34 +/- 1.58 ng/ml) and a slight increment in HGH peaks after insulin (from 23.09 +/- 7.19 to 31.46 +/- 7.95 ng/ml) were observed following reduction in plasma TG. Arginine test was carried out in 7 normal subjects during saline infusion and at the 3rd hour of lipid infusion (Intralipid 20%). HGH response to arginine was absent in all of the subjects during lipid infusion. The HGH response to insulin test, carried out in 9 other normal subjects during saline infusion and at the 3rd hour of lipid infusion (Lipiphysan 15%) was significantly inhibited during lipid infusion. Since lipid infusion provoked an increment, not only in plasma TG but also in FFA, the inhibition of HGH release could be correlated with the elevated plasma levels of both TG and FFA. The results obtained in both spontaneous and experimental hyperlipemia not only confirm the role played by FFA in the regulation of HGH secretion, but also support the hypothesis that elevated TG levels could inhibit HGH response to some stimuli.     

Euthyroid hyperthyrotropinemia secondary to hyperestrogenemia in a male with congenital adrenal hyperplasia.

A 10-year-old boy with congenital adrenal hyperplasia and associated hyperplastic testicular adrenal rests had high serum concentrations of 17-OH progesterone (17-OHP), estradiol (E2), testosterone (T), and basal and TRH-stimulated TSH and PRL, but normal thyroid hormones (T3, T4, FT3, FT4) and thyroxine-binding globulin (TBG). Upon dexamethasone therapy, steroid hormones returned progressively toward normal as did both PRL and TSH; PRL declined faster than TSH. Serum E2 correlated better with PRL than with TSH. Therefore, the responsiveness of the thyrotrophs to the ambient concentration of E2 is lower and slower than that of the lactotrophs. In the context of the inconclusive data on the role of estrogens in controlling the secretion of TSH in humans, our case suggests that E2 does stimulate the secretion of basal and TRH-elicited both TSH and PRL, and that this positive action is unopposed by T. In contrast, T antagonizes the estrogen-induced increase in serum TBG. We also postulate that E2 might impair the bioactivity of TSH, in order to explain (i) the approximate 3-fold increase in serum TSH coexisting with a normally sized (rather than enlarged) thyroid and normal (rather than increased) serum thyroid hormones, and (ii) the inability of TRH-stimulated TSH to acutely raise FT3 serum levels.     

Effect of pharmacological doses of oxytocin on insulin response to glucose in normal man

In this study we have examined the effect of the administration of oxytocin on basal blood concentrations of insulin, glucagon, cortisol, growth hormone, and on the dynamic secretory response of these hormones to intravenous glucose administration (0.33 g/kg) in basal condition and after the injection of 3 IU (1 plus 2 IU/1 h) or 6 IU (2 plus 4 IU/1 h) of oxytocin (6 subjects for each group). The highest dose of oxytocin (6 IU) used significantly increased insulin secretion in response to intravenously administered glucose. No significant change of insulin secretion was observed with 3 IU of oxytocin. Glucagon, cortisol, and growth hormone response to intravenous injection of glucose was not affected by oxytocin (3 or 6 IU) administration. These results suggest that high doses of oxytocin affect beta-cell function in normal man.     

Effects of parathyroid hormone on cyclic AMP, cyclic GMP, and efflux of calcium in isolated renal tubules.

The effects of parathyroid hormone (PTH) on concentrations of cyclic AMP and cyclic GMP were investigated in isolated renal cortical tubules from hamsters. Efflux of 45Ca from tubules was compared to temporal changes in both cyclic nucleotide concentrations. A rapid increase in cyclic AMP occurred following addition of PTH which was maximal by 1 min but decreased over the next 4 min period. Cyclic GMP concentrations were not significantly altered at 1 min but increased between 1 and 5 min from basal levels. Concentrations of both nucleotides remained significantly elevated from basal levels between 5 and 15 min following PTH. Efflux of 45Ca was increased by PTH with time-course changes closely paralleling changes in cyclic GMP concentrations. Changes in both cyclic AMP and cyclic GMP were related to PTH concentrations of the incubation media and were increased by addition of theophylline. Increasing the calcium concentration from 1 to 3 mM did not significantly alter the effect of PTH on cyclic AMP, however, cyclic GMP concentrations were further increased.     

Changes in the exercise-induced hormone response to branched chain amino acid administration.

It was the aim of the present experiment to detect possible effects of branched-chain amino acids (BCAA) on the endocrine response to 1 h of continuous running. Blood samples were collected from 14 long-distance runners (age 24-42 years) in two different trials performed at 1-week intervals. In both trials (E and P) blood samples were collected at the following times: 9 a.m. (basal values sample), 10.30 a.m. (sample 90), 11.30 a.m. (sample 150), 12.30 p.m. (sample 210); the athletes performed 1 h of running at a constant predetermined speed between samples 90 and 150. Following the basal sample a mixture containing BCAA (E trial), or not containing BCAA (P trial) was ingested. In both trials no hormone basal concentrations, except insulin, were changed before exercise. In P trial, following exercise (sample 150), human growth hormone (HGH), prolactin (PRL), adrenocorticotropic hormone (ACTH) and cortisol (C) increased, while testosterone (T) decreased. In sample 210, after 1 h of rest, while ACTH, PRL and HGH had recovered to basal concentrations, C remained elevated and T displayed a further decrease. In the E trial a similar pattern of change was observed in sample 150 for HGH, PRL, ACTH and C; in sample 210 HGH and PRL displayed significantly lower values than in the corresponding P trial samples. The T was not modified by the running exercise and increased during the recovery period. It is, therefore, suggested that BCAA administration before exercise affects the response of some anabolic hormones, mainly HGH and T.     

Decreased levels of ionized calcium one year after hemithyroidectomy: importance of reduced thyroid hormones

BACKGROUND: Previously we have found reduced levels of total serum calcium and 1,25(OH)2D3 despite an unaltered stimulated parathyroid hormone (PTH) secretion 1 year after hemithyroidectomy. The present study was undertaken to elucidate the possible relationship between calcium homeostasis, thyroid hormones and bone resorption in a group of 45 consecutive patients subjected to hemithyroidectomy because of a solitary nodule. All patients had free T4 and T3 levels within normal range preoperatively. METHODS: Thyroid hormones, bone mineral and biochemical variables known to reflect calcium homeostasis were studied. Patients were divided into three separate groups depending on their pre- and postoperative thyroid hormone status. RESULTS: One year postoperatively, serum levels of free T4 were decreased and that of thyrotropin (TSH) increased in the entire group of patients. The concentration of ionized calcium was reduced from 1.25 +/- 0.05 to 1.22 +/- 0.04 (p < 0.001) despite an unaltered PTH value (2.8 +/- 1.0 vs. 3.1 +/- 1.5, p = 0.50). A significant reduction in C-terminal telopeptide type 1 collagen (1CTP) indicated decreased bone resorption 1 year after surgery (p < 0.05). Subgroup analysis showed that a reduction in ionized calcium was seen only among patients with a postoperative decrease in free T4. Patients with subclinical hyperthyroidism preoperatively presented the lowest postoperative levels of ionized calcium, significantly reduced levels of 1CTP and increased levels of phosphate and creatinine. Multiple linear regression analysis showed that age (p < 0.05) and postoperatively changed serum levels of TSH (p < 0.05), creatinine (p < 0.05), phosphate (p < 0.001) and FT4 (p < 0.01) were independently associated with altered levels of ionized calcium. CONCLUSION: We conclude that the reduction in ionized calcium 1 year after hemithyroidectomy was not due to PTH deficiency. Instead our results suggest that the reduced effects of thyroid hormones on bone and kidney function is essential.     

Peripheral effects of PTH are not altered after thyroid surgery in euthyroid patients

BACKGROUND: We have previously found decreased serum levels of both ionized calcium and 1,25(OH)2D and an increase in serum phosphate levels at 1 year after hemithyroidectomy. However, basal and stimulated parathyroid hormone (PTH) secretions were not altered. To investigate whether the observed biochemical changes after unilateral thyroid surgery may be due to a relative end-organ resistance to PTH, we studied the peripheral effects of infused hPTH-(1-34) in 6 patients preoperatively and 3 months after hemithyroidectomy. METHODS: Serum levels of TSH, FT4 and FT3 were measured pre- and postoperatively. hPTH-(1-34) was infused at 0.9 IU/kg/h during 6 h. Blood samples for analysis of ionized calcium, intact PTH, phosphate, 25(OH)D, 1,25(OH)2D and urinary samples for calcium, phosphate and nephrogenous(n)-cAMP analysis were taken at baseline, when the infusion was discontinued after 6 h and at 24 h. RESULTS: Three months after hemithyroidectomy, serum levels of FT3 were decreased and TSH levels increased. Pre- and postoperative hPTH-(1-34) infusions induced increased serum levels of ionized calcium, 1,25(OH)2D, increased urinary excretion of phosphate and elevated n-cAMP levels. The changes in the studied biochemical variables during the hPTH-(1-34) infusions did not differ between the two study occasions. CONCLUSION: By using a 6-hour hPTH-(1-34) infusion protocol, we have shown that the peripheral PTH effect is not altered by a slight reduction in thyroid hormone levels at 3 months after hemithyroidectomy.     

Mechanisms of insulin inhibition of ACTH-stimulated steroid secretion by cultured bovine adrenocortical cells.

Results of previous studies indicated that insulin at levels comparable to those in humans during hyperinsulinemia decreased ACTH-stimulated cortisol and androstenedione secretion by bovine adrenal fasciculata-reticularis cells in primary culture. In the present studies this inhibitory action was examined further by comparing the effects of insulin on ACTH-stimulated corticosteroid secretion with its effects on 8-(4-chlorophenylthio)-cAMP (cpt-cAMP), forskolin- and [5val]angiotensin II (Ang II)-stimulated corticosteroid secretion. Effects on corticosteroid secretion were correlated with effects on cAMP accumulation and rates of cAMP production. Monolayers were incubated for 24 h in the absence or presence of each agonist alone or in combination with insulin. Insulin (1.7 x 10(-9) or 17.5 x 10(-9) M) caused about a 50% decrease in cortisol and androstenedione secretion in response to ACTH (10(-11) or 10(-8) M). Insulin also decreased ACTH-stimulated aldosterone secretion by cultured glomerulosa cells. Cpt-cAMP (10(-4) or 10(-3) M)-stimulated increases in cortisol and androstenedione secretion were inhibited by insulin, but to a lesser extent than those in response to ACTH. The inhibition of cpt-cAMP-stimulated steroid secretion was not related to increased degradation of the cyclic nucleotide. Increases in cortisol and androstenedione secretion caused by a submaximal concentration (10(-6) M) of forskolin were decreased 50-70% by insulin. In contrast, insulin failed to significantly affect cortisol or androstenedione secretion caused by a maximal concentration (10(-5) M) of forskolin. The secretory responses to Ang II (10(-8) M) were also unaffected by insulin. The effect of insulin to inhibit ACTH-stimulated steroid secretion was accompanied by a reduction in cAMP accumulation as well as an apparent inhibition of adenylate cyclase activation. These data indicate that the effect of insulin to attenuate ACTH-stimulated corticosteroid secretion results from both an inhibition of ACTH-stimulated adenylate cyclase activity and an antagonism of the intracellular actions of cAMP.     

Seasonal changes in some plasma hormones in pigeons from different environments

Seasonal variation in the plasma concentration of lutropin (LH), follitropin (FSH), prolactin (PRL), thyroxine (T4), triiodothyronine (T3) and corticosterone (B) were measured in the pigeon by RIA methods. Pigeons were maintained indoors under constant ambient temperature (Ta) and simulated natural daylight (LD), 12:12 L:D regimens or outdoors exposed to seasonal variations in temperature and photoperiod at Oulu, Finland. Only slight changes of gonadotropins (LH, FSH) were observed throughout the year, without any clear photosensitive or photorefractory period. In the indoor (natural LD) group, LH stayed elevated from May until October. Interdependence between the circannual hormonal fluctuation and photoperiod could not be shown, although the amplitude of FSH, T4 and T3 fluctuation of pigeons maintained in laboratory conditions were greater than that of natural LD and outdoor pigeons, whose circannual rhythms were similar. A high concentration of plasma PRL in autumn and the peak value of B in winter for all groups are thought to be correlated to lipid metabolism. Two peaks, the first in winter and the second in autumn, observed in T4 and T3 hormone profiles, may be due to molting of the pigeons.