Derived
structure parameters that describe constant properties of the overall
feedback control system may add useful information for special purposes, e.g. in diagnosis of
nonthyroidal illness syndrome or
central hypothyroidism.
Secretory capacity (GT) Thyroid's secretory capacity (
GT, also referred to as SPINA-GT) is the maximum stimulated amount of thyroxine the thyroid can produce in one second.
GT is elevated in hyperthyroidism and reduced in hypothyroidism.
GT is calculated with \hat G_T = {{\beta _T (D_T + [TSH])(1 + K_{41} [TBG] + K_{42} [TBPA])[FT_4 ]} \over {\alpha _T [TSH]}} or \hat G_T = {{\beta _T (D_T + [TSH])[TT_4 ]} \over {\alpha _T [TSH]}} \alpha _T: Dilution factor for T4 (reciprocal of apparent volume of distribution, 0.1 l−1) \beta _T: Clearance exponent for T4 (1.1e-6 sec−1)
K41: Dissociation constant T4-TBG (2e10 L/mol)
K42: Dissociation constant T4-TBPA (2e8 L/mol)
DT: EC50 for TSH (2.75 mU/L)
GD is obtained with \hat G_D = {{\beta _{31} (K_{M1} + [FT_4 ])(1 + K_{30} [TBG])[FT_3 ]} \over {\alpha _{31} [FT_4 ]}} or \hat G_D = {{\beta _{31} (K_{M1} + [FT_4 ])[TT_3 ]} \over {\alpha _{31} [FT_4 ]}} \alpha _{31}: Dilution factor for T3 (reciprocal of apparent volume of distribution, 0.026 L−1) \beta _{31}: Clearance exponent for T3 (8e-6 sec−1)
KM1: Dissociation constant of type-1-deiodinase (5e-7 mol/L)
K30: Dissociation constant T3-TBG (2e9 L/mol) It is reduced in thyrotropic insufficiency Somewhat similar to the TSH Index it is calculated from equilibrium values for TSH and FT4, however with a different equation.
TFQI The
Thyroid Feedback Quantile-based Index (TFQI) is another parameter for thyrotropic pituitary function. It was defined to be more robust to distorted data than JTI and TTSI. It is calculated with TFQI = F_{FT4}(FT4) - (1 - F_{TSH}(TSH)) from quantiles of FT4 and TSH concentration (as determined based on
cumulative distribution functions). Per definition the TFQI has a
mean of 0 and a
standard deviation of 0.37 in a reference population. TFQI results are also elevated in
takotsubo syndrome, potentially reflecting type 2
allostatic load in the situation of
psychosocial stress. Reductions have been observed in subjects with
schizophrenia after initiation of therapy with
oxcarbazepine, potentially reflecting declining allostatic load.
Reconstructed set point In healthy persons, the intra-individual variation of TSH and thyroid hormones is considerably smaller than the inter-individual variation. This results from a personal
set point of thyroid homeostasis. In hypothyroidism, it is impossible to directly access the set point, but it can be reconstructed with methods of systems theory. A computerised algorithm, called Thyroid-SPOT, which is based on this mathematical theory, has been implemented in software applications. In patients undergoing thyroidectomy it could be demonstrated that this algorithm can be used to reconstruct the personal set point with sufficient precision. ==Effects of drugs==