In order to evaluate the effects of climatic factors on the secretion of thyroid hormones and TSH in a high latitude population, we have taken serum and urine samples from 20 healthy men from northern Finland (67 -68 N) every 2 months for a period of 14 months. Serum free triiodothyronine (T 3 ) levels were lower in February than in August (3·9 vs 4·4 pmol/l, P<0·05) and TSH levels were higher in December than during other months (2·1 vs 1·5-1·7 mU/l, P<0·01). Serum total and free thyroxine (T 4 ), total T 3 and reverse T 3 levels and urinary T 4 levels were unchanged. Urinary T 3 levels were significantly higher in winter than in summer. Serum free T 3 correlated highly significantly with the outdoor temperature integrated backwards weekly for 7-56 days (r=0·26 for 1-56 days) from the day when the blood samples were taken. Serum TSH did not show any significant correlation with the thyroid hormones or with the integrated temperature of the previous days, but it did show an inverse and significant correlation (r= 0·31) with the ambient luminosity integrated backwards for 7 days from the day when the blood sample was taken. The gradually increasing correlation between outdoor temperatures and serum free T 3 suggests that the disposal of thyroid hormones is accelerated in winter, leading to low serum free T 3 levels and a high urinary free T 3 excretion. Since there was no correlation between thyroid hormones and serum TSH, the feedback mechanism between TSH and thyroid hormones may not be the only contributing factor, and other factors such as ambient luminosity may at least partly determine serum TSH in these conditions. Also urinary free T 3 appears to be a novel and non-invasive indicator for thyroid physiology.

Alpha-tocopherol, albumin and selenium may play a role in the low mortality from CHD observed in northernmost Finland. The favourable serum antioxidant status in northerners may be credited to the local diet.

No abstract

To clarify how seasonal changes of environmental luminosity affect melatonin secretion in human beings, nocturnal urine samples were collected every 2 months throughout the year from 20 outdoor workers living in an area, for which exact data on solar irradiance and temperature were available (67-68 degrees N). Melatonin secretion rates, determined by melatonin-specific radioimmunoassays, were found to be twice as high in December as in April, i.e. 0.88 +/- 0.16 nmol/12 hr (mean +/- S.E.) versus 0.43 +/- 0.09 nmol/12 hr, P < 0.05. Regression analyses showed that melatonin values correlated best inversely with solar irradiance reflected off the ground (albedo). A 10-fold increase in albedo for 1-4 weeks prior to the date of urine sampling was associated with a ca. 50% reduction in melatonin secretion. Association with global irradiance was weaker and did not correlate with temperature. Thus, light reaching the eyes via snow or other reflecting surfaces appears to be most effective. These results assist us in understanding some of the mechanisms involved in certain biological phenomena that exhibit seasonal variations as, for instance, reproduction and self-destructive behavior.

Objectives. We set out to find out in which way seasonal changes of environmental luminosity could affect melatonin secretion in humans. Study design. For an entire year we collected every two months nocturnal urine samples from 20 male outdoor workers who lived and worked in an area of the circumpolar region from which exact data of solar irradiance and temperature were available. Methods. Melatonin secretion rates were assessed with our melatonin-specific radioimmunoassay. Results. Melatonin secretion was twice as high in December as in April, 0.88 ± 16 nmol/12 h (mean ± SE) vs. 0.43 ± 9 nmol/12 h, p < 0.05. Regression analyses showed that melatonin values best correlated inversely with solar irradiance reflected off the ground (the so-called 'albedo'). A tenfold increase in albedo for 1-4 weeks before the urine samplings were collected was associated with a ca. 50% reduction in melatonin secretion. The association with global irradiance was weaker and none existed with temperature or other environmental variables. Discussion. Light reaching the eyes via the snow or other reflecting surfaces appears to be most effective. These results help us to understand some of the mechanisms involved in certain biological phenomena that exhibit seasonal variations such as reproductive and selfdestructive behaviours.