Endocrine Abstracts

JOINT3984

Background: Habitual cold exposure is mandatory to hunters living in East Greenland. Hunters in Greenland depend on the ability to produce heat. Brown Adipose Tissue has the capacity to produce heat by activation of Uncoupling Protein-1 (UCP1). Upregulation of UCP1 for activating BAT depends on triiodothyronine (T3) in concert with sympathetic stimulation. Hence, these hunters provide a unique model for intervention on the effect of blocking the sympathetic system during cold exposure.

Aim: We aimed to explore body temperature and thyroid hormones while blocking sympathetic activity in Greenlanders with habitual cold exposure.

Methods: We studied Greenlandic hunters (n = 7; healthy men) in East Greenland for 10 days during winter (February). The intervention comprised of a non-selective beta-blocker (Propranolol; 160 mg) taken daily for seven consecutive days. Blood samples were drawn daily for measurement of thyrotropin (TSH) and thyroid hormone levels, and body temperature was measured morning (AM) and evening (PM). Hunters reported the daily number of hours spent outdoor. Body temperature, TSH, fT3, and fT4 were compared prior to, and following intervention.

Results: Hunters spent an average of 5. 2 hours outdoor, and the body temperature was 35. 8 ^oC AM and 36. 2^oC PM. Body temperature decreased with intervention for PM measurements (36. 66^oC; 35. 76^oC), while not for AM measurements (35. 84^oC; 35. 99^oC). The circadian difference was lower with intervention (0. 82^oC; 0. 26^oC). Both PM temperature and circadian difference associated with the number of hours spent outdoors, with the latter explaining 18% of the circadian difference in body temperature. Average TSH was 2. 31 before and 3. 53 mIU/l with intervention, and fT3 was 3. 14 without and 2. 91 pmol/l with intervention. The pre-to-post intervention difference of 0. 61^oC for PM temperature was seen in parallel with a decrease of 0. 23 pmol/l in fT3 and rise of 1. 22 mIU/l in TSH.

Conclusion: Body temperature was stable with more than 5 hours spent outdoor in the Arctic winter. Intervention with a beta-blocker was followed by changes in body temperature in the evening while not in the morning. Changes in thyroid hormone levels were in keeping with the anticipated role of adrenergic stimulation of the thyroid gland. The rise in pituitary stimulation of the thyroid may be a response to decreased adrenergic drive on the thyroid. The results illustrate the complex interaction by thyroid hormones and adrenergic system to uphold body temperature.