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. 2022 Jun 9;13:825107. doi: 10.3389/fendo.2022.825107

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Copyright © 2022 Hoermann, Pekker, Midgley, Larisch and Dietrich

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Combining mechanisms of FT3 homeostasis and FT3 adaptation. (A) In system (sys. 9), with k32 = 1, k423 = 0.5, k42 = 0 and other parameters =1, increasing k43 (the FT3 production rate constant of peripheral organs) increases the level of FT3 (green curve), while the level of FT4 decreases (blue curve). FT3 and FT4 move in opposite directions. This mechanism disjoins the levels of FT3 and FT4. (B) The level of FT3 is protected by this mechanism in system (sys. 9) within a certain range, as both k43 = 0-1 (as in A) and k32 (FT4 production rate constant) vary simultaneously. (C) In system (sys. 10), the TSH feedforward on FT3 is switched on (k42 = 1). This mechanism increases the level of FT3 more strongly, as k32 decreases, compared to B (sys. 9, k42 = 0). (D) In system (sys. 10), decreasing the range of s1 (TRH production rate constant) (here s1 = 0.5 to 0.1, with other parameters as in (C) shifts and reduces the range of FT3 levels, compared to C. Color code of hormones: TSH black, FT4 blue, FT3 green.