Thyroid function tests

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][4] The new techniques were more accurate, leading to the second, third, and even fourth generations of TSH assay, with each generation possessing ten times greater functional sensitivity than the last.[7] Third generation immunometric assay methods are typically automated.[3] Fourth generation TSH immunometric assay has been developed for use in research.[4]

Third generation TSH assay is the current requirement for modern standards of care. At present, TSH testing in the United States is typically carried out with automated platforms using advanced forms of immunometric assay.[3] Nonetheless, there is currently no international standard for measurement of thyroid-stimulating hormone.[4]

Accurate interpretation takes a variety of factors into account, such as the thyroid hormones i.e. thyroxine (T₄) and triiodothyronine (T₃), current medical status (such as pregnancy[3]),[4] certain medications like propylthiouracil,[4] temporal effects including circadian rhythm[8] and hysteresis,[9] and other past medical history.[10]

Total thyroxine is rarely measured, having been largely superseded by free thyroxine tests. Total thyroxine (Total T₄) is generally elevated in hyperthyroidism and decreased in hypothyroidism.[2] It is usually slightly elevated in pregnancy secondary to increased levels of thyroid binding globulin (TBG).[2]

Total T4 is measured to see the bound and unbound levels of T4. The total T4 is less useful in cases where there could be protein abnormalities. The total T4 is less accurate due to the large amount of T4 that is bound. The total T3 is measured in clinical practice since the T3 has decreased amount that is bound as compared to T4.

Reference ranges depend on the method of analysis. Results should always be interpreted using the range from the laboratory that performed the test. Example values are:

Free thyroxine (fT₄ or free T4) is generally elevated in hyperthyroidism and decreased in hypothyroidism.[2]

Reference ranges depend on the method of analysis. Results should always be interpreted using the range from the laboratory that performed the test. Example values are:

Total triiodothyronine (Total T₃) is rarely measured, having been largely superseded by free T3 tests. Total T3 is generally elevated in hyperthyroidism and decreased in hypothyroidism.[2]

Reference ranges depend on the method of analysis. Results should always be interpreted using the range from the laboratory that performed the test. Example values are:

Free triiodothyronine (fT₃ or free T3) is generally elevated in hyperthyroidism and decreased in hypothyroidism.[2]

Reference ranges depend on the method of analysis. Results should always be interpreted using the range from the laboratory that performed the test. Example values are:

An increased thyroxine-binding globulin results in an increased total thyroxine and total triiodothyronine without an actual increase in hormonal activity of thyroid hormones.

Reference ranges:

• Transthyretin (prealbumin)
• Albumin

Thyroid hormone uptake (T_uptake or T_{3 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. It is not directly related to triiodothyronine, despite the name T_{3 uptake}.[2]

Reference ranges:

• Thyroid Hormone Binding Ratio (THBR)
• Thyroxine-binding index (TBI)

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] This test is rarely used now that reliable free thyroxine and free triiodothyronine assays are routinely available.

FTI is elevated in hyperthyroidism and decreased in hypothyroidism.[2]

Thyroid's secretory capacity (G_T, also referred to as SPINA-GT) is the maximum stimulated amount of thyroxine the thyroid can produce in one second.[24] G_T is elevated in hyperthyroidism and reduced in hypothyroidism.[25]

G_T is calculated with

${\displaystyle {\hat {G}}_{T}={{\beta _{T}(D_{T}+[TSH])(1+K_{41}[TBG]+K_{42}[TBPA])[FT_{4}]} \over {\alpha _{T}[TSH]}}}$

or

${\displaystyle {\hat {G}}_{T}={{\beta _{T}(D_{T}+[TSH])[TT_{4}]} \over {\alpha _{T}[TSH]}}}$

${\displaystyle \alpha _{T}}$: Dilution factor for T4 (reciprocal of apparent volume of distribution, 0.1 l⁻¹)
${\displaystyle \beta _{T}}$: 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)
D_T: EC₅₀ for TSH (2.75 mU/L)[24]

The sum activity of peripheral deiodinases (G_D, also referred to as SPINA-GD) is reduced in nonthyroidal illness with hypodeiodination.[21][22][26]

G_D is obtained with

${\displaystyle {\hat {G}}_{D}={{\beta _{31}(K_{M1}+[FT_{4}])(1+K_{30}[TBG])[FT_{3}]} \over {\alpha _{31}[FT_{4}]}}}$

or

${\displaystyle {\hat {G}}_{D}={{\beta _{31}(K_{M1}+[FT_{4}])[TT_{3}]} \over {\alpha _{31}[FT_{4}]}}}$

${\displaystyle \alpha _{31}}$: Dilution factor for T3 (reciprocal of apparent volume of distribution, 0.026 L⁻¹)
${\displaystyle \beta _{31}}$: Clearance exponent for T3 (8e-6 sec⁻¹)
K_M1: 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.[26]

It is calculated with

${\displaystyle TSHI=LN(TSH)+0.1345*FT4}$.

Additionally, a standardized form of TSH index may be calculated with

${\displaystyle sTSHI=(TSHI-2.7)/0.676}$.[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.[28][29] Somewhat similar to the TSH Index it is calculated from equilibrium values for TSH and FT4, however with a different equation.

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

${\displaystyle TFQI=F_{FT4}(FT4)-(1-F_{TSH}(TSH))}$

from quantiles of FT4 and TSH concentration (as determined based on cumulative distribution functions).[30] Per definition the TFQI has a mean of 0 and a standard deviation of 0.37 in a reference population.[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.[38]

In healthy persons, the intra-individual variation of TSH and thyroid hormones is considerably smaller than the inter-individual variation.[39][40][41] This results from a personal set point of thyroid homeostasis.[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.[48]

The Centers for Disease Control and Prevention has published the following laboratory procedure manuals for measuring thyroid-stimulating hormone:

• Thyroid Stimulating Hormone (TSH) (University of Washington Medical Center). September 2011. Method: Access 2 (Beckman Coulter).
• Thyroid Stimulating Hormone (TSH) (Collaborative Laboratory Services). September 2011. Method: Access 2 (Beckman Coulter).
• Thyroid Stimulating Hormone (TSH). September 2009. Method: Access 2 (Beckman Coulter).
• Lab 18 Thyroid Stimulating Hormone. 2001-2002. Method: Microparticle Enzyme Immunoassay.
• Lab 18 TSH - Thyroid Stimulating Hormone. 1999-2000. Method: Microparticle Enzyme Immunoassay.