Thyroid function tests
It is also requested routinely in conditions linked to thyroid disease, such as atrial fibrillation and anxiety disorder.
Thyroid-stimulating hormone (TSH, thyrotropin) is generally increased in hypothyroidism and decreased in hyperthyroidism,[2] making it the most important test for early detection of both of these conditions.
[3][4] The result of this assay is suggestive of the presence and cause of thyroid disease, since a measurement of elevated TSH generally indicates hypothyroidism, while a measurement of low TSH generally indicates hyperthyroidism.
The production of TSH is controlled by thyrotropin-releasing hormone (TRH), which is produced in the hypothalamus.
[citation needed] First-generation TSH assays were done by radioimmunoassay and were introduced in 1965.
[3] There were variations and improvements upon TSH radioimmunoassay, but their use declined as a new immunometric assay technique became available in the middle of the 1980s.
[3] Fourth generation TSH immunometric assay has been developed for use in research.
[4] Third generation TSH assay is the requirement for modern standards of care.
TSH testing in the United States is typically carried out with automated platforms using advanced forms of immunometric assay.
[2] It is usually slightly elevated in pregnancy secondary to increased levels of thyroid binding globulin (TBG).
The total T3 is measured in clinical practice since the T3 has decreased amount that is bound as compared to T4.
[citation needed] Reference ranges depend on the method of analysis.
Reference ranges: Reference ranges: Thyroid hormone uptake (Tuptake or T3 uptake) is a measure of the unbound thyroxine binding globulins in the blood, that is, the TBG that is unsaturated with thyroid hormone.
[2] Unsaturated TBG increases with decreased levels of thyroid hormones.
[2] Reference ranges: The Free Thyroxine Index (FTI or T7) is obtained by multiplying the total T4 with T3 uptake.
[2] FTI is considered to be a more reliable indicator of thyroid status in the presence of abnormalities in plasma protein binding.
[2] 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.
: 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)[24] The sum activity of peripheral deiodinases (GD, also referred to as SPINA-GD) is reduced in nonthyroidal illness with hypodeiodination.
: 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)[24] Jostel's TSH index (JTI or TSHI) helps to determine thyrotropic function of anterior pituitary on a quantitative level.
[27] It is reduced in thyrotropic insufficiency[27] and in certain cases of non-thyroidal illness syndrome.
[27] The Thyrotroph Thyroid Hormone Sensitivity Index (TTSI, also referred to as Thyrotroph T4 Resistance Index or TT4RI) was developed to enable fast screening for resistance to thyroid hormone.
The Thyroid Feedback Quantile-based Index (TFQI) is another parameter for thyrotropic pituitary function.
from quantiles of FT4 and TSH concentration (as determined based on cumulative distribution functions).
[30] Higher values of TFQI are associated with obesity, metabolic syndrome, impaired renal function, diabetes, and diabetes-related mortality.
[30][31][32][33][34][35][36] TFQI results are also elevated in takotsubo syndrome,[37] 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.
[42] In hypothyroidism, it is impossible to directly access the set point,[43] but it can be reconstructed with methods of systems theory.
[44][45][46] A computerised algorithm, called Thyroid-SPOT, which is based on this mathematical theory, has been implemented in software applications.
[47] In patients undergoing thyroidectomy it could be demonstrated that this algorithm can be used to reconstruct the personal set point with sufficient precision.
↓: reduced serum concentration or structure parameter; ↑: increased serum concentration or structure parameter; ↔: no change; TSH: Thyroid-stimulating hormone; T3: Total triiodothyronine; T4: Total thyroxine; fT4: Free thyroxine; fT3: Free triiodothyronine; rT3: Reverse triiodothyronine The Centers for Disease Control and Prevention has published the following laboratory procedure manuals for measuring thyroid-stimulating hormone:
