Endocrine Abstracts

Background: Thyroid hormones interact with sympathetic stimulation for activating Brown Adipose Tissue (BAT) during cold exposure. Triiodothyronine (T3) upregulates the expression of Uncoupling Protein-1 (UCP1), a protein in BAT essential for heat production. Studies of human cold exposure have demonstrated both increased production and clearance of T3. Greenlandic hunters are characterized by mandatory chronic cold exposure, and cold adaptation is inevitable. Hence, they provide an exquisite model for evaluating metabolic effects of cold exposure.

Aim: We aimed to explore the dynamics of thyroid hormones when blocking sympathetic activity in Greenlandic hunters during winter to explore mechanisms of brown adipose tissue activation.

Methods: We conducted a 10-day study of Greenlandic hunters (n = 7) in East Greenland during February. The sympathetic system was blocked using a non-selective beta blocker for 7 consecutive days. A group of non-hunter Greenlanders (n = 8) from the same settlement were included for comparison. All participants were healthy men. Blood samples were drawn daily for measurement of TSH and thyroid hormone levels and thyroglobulin in serum (s-Tg).

Results: At baseline, TSH was higher (median 1.99 mIU/l vs. 1.76 mIU/l), T3 lower (median 3.27 pg/mL vs 3.45 pg/mL), and s-Tg higher (median 18.44 µg/l vs 12 µg/l) among hunters compared to controls. After blocking the sympathetic system, s-Tg decreased initially and recovered to pre-blocking level following a marked rise in TSH (median 3.09 mIU/l) among the cold exposed hunters.

Conclusion: The dynamics of thyroid hormones show different patterns in the hunter group than the control prior to intervention as well as after intervention. When blocking the sympathetic nervous system, TSH increases followed by recovery of s-Tg to the pre-blocking level. A possible explanation is that when blocking the activation of BAT via the sympathetic system, TSH increases to uphold the production of T3, needed for maintaining BAT activity and the obligatory heat production among cold exposed hunters in East Greenland during winter.