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. 2024 Dec 1;14(1):e240272.
doi: 10.1530/ETJ-24-0272. Online ahead of print.

Thyroid response to blocking sympathetic activity in chronic cold exposed hunters in East Greenland: a case-control study

Mette Motzfeldt Jensen  1 , Charlotte Almasi  2 , Hans Christian Florian Sørensen  3 , Stine Linding Andersen  4 , Stig Andersen  5
Affiliations

Thyroid response to blocking sympathetic activity in chronic cold exposed hunters in East Greenland: a case-control study

Mette Motzfeldt Jensen et al. Eur Thyroid J. .

Abstract

Background: Thyroid hormones and sympathetic stimulation are needed for activating Brown adipose tissue (BAT) during cold exposure. Studies of human cold exposure have demonstrated both increased production and raised clearance of triiodothyronine (T3). Greenlandic hunters provide a unique 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 inspire knowledge on mechanisms of BAT activation.

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

Results: Hunters had higher serum thyroglobulin, TSH, and high fT3/fT4 ratio compared to controls. Blocking the sympathetic activity was followed by changes in serum thyroglobulin and fT3 with an initial decrease and subsequent restoration of levels, while TSH and fT4 showed a gradual increase over the course of the study. The fT3/fT4 ratio showed a continuous and marked decrease.

Conclusion: We hypothesise that when blocking the sympathetic system, TSH increases to uphold the production of T3 needed for maintaining BAT activity. Additionally, alterations of fT3/fT4-ratio support a hypothesis of adrenergic stimulation promoting T3 over T4 secretion from the thyroid via the adrenergic nerve terminals in the thyroid.

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Conflict of interest statement

The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the work.

Figures

Figure 1
Figure 1
A hunter on his dinghy boat in February in East Greenland illustrating the cold outdoor working conditions.
Figure 2
Figure 2
Median thyroglobulin, thyrotropin (TSH), free T3, free T4 in serum and fT3/fT4 ratio among hunters exposed to cold by trade (black bar) prior to (full bars) and after 7 days (striped bars) of daily beta blockage with propranolol 160 mg and controls (grey bar) having indoor occupation and living in the same settlement in East Greenland.
Figure 3
Figure 3
Thyroglobulin, thyrotropin (TSH), free T3 and free T4 levels (median) in serum among hunters (solid line) receiving a non-specific beta-blocker (intervention) and controls (squared dots) without intervention. All lived in the same settlement in East Greenland and were investigated in winter.
Figure 4
Figure 4
Ratio of fT3/fT4 among hunters receiving intervention (solid line) and controls living in the same settlement in East Greenland (squared dots). The intervention consisted of a non-specific beta-blocker from day-1 through day-7.
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