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Rudolf Hörmann, John Edward M. Midgley, Rolf Larisch, Johannes Wolfgang Christian Dietrich

• \(\textit {Background/Aim:}\) Operating far from its equilibrium resting point, the thyroid gland requires stimulation via feedback-controlled pituitary thyrotropin (TSH) secretion to maintain adequate hormone supply. We explored and defined variations in the expression of control mechanisms and physiological responses across the euthyroid reference range. \(\textit {Methods:}\) We analyzed the relational equilibria between thyroid parameters defining thyroid production and thyroid conversion in a group of 271 thyroid-healthy subjects and 86 untreated patients with thyroid autoimmune disease. \(\textit {Results:}\) In the euthyroid controls, the \(FT_{3}\)-\(FT_{4}\) (free triiodothyronine-free thyroxine) ratio was strongly associated with the \(FT_{4}\)-TSH ratio (tau = –0.22, p < 0.001, even after correcting for spurious correlation), linking \(T_{4}\) to \(T_{3}\) conversion with TSH-standardized \(T_{4}\) production. Using a homeostatic model, we estimated both global deiodinase activity and maximum thyroid capacity. Both parameters were nonlinearly and inversely associated, trending in opposite directions across the euthyroid reference range. Within the panel of controls, the subgroup with a relatively lower thyroid capacity (<2.5 pmol/s) displayed lower \(FT_{4}\) levels, but maintained \(FT_{3}\) at the same concentrations as patients with higher functional and anatomical capacity. The relationships were preserved when extended to the subclinical range in the diseased sample. \(\textit {Conclusion:}\) The euthyroid panel does not follow a homogeneous pattern to produce random variation among thyroid hormones and TSH, but forms a heterogeneous group that progressively displays distinctly different levels of homeostatic control across the euthyroid range. This suggests a concept of relational stability with implications for definition of euthyroidism and disease classification.