Thyroid function tests

Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid.[1] TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy. It is also requested routinely in conditions linked to thyroid disease, such as atrial fibrillation and anxiety disorder.

Thyroid function tests
MeSHD013960
MedlinePlus003444

A TFT panel typically includes thyroid hormones such as thyroid-stimulating hormone (TSH, thyrotropin) and thyroxine (T4), and triiodothyronine (T3) depending on local laboratory policy.

Thyroid-stimulating hormone

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.[2] However, when TSH is measured by itself, it can yield misleading results, so additional thyroid function tests must be compared with the result of this test for accurate diagnosis.[4][5][6]

TSH is produced in the pituitary gland. The production of TSH is controlled by thyrotropin-releasing hormone (TRH), which is produced in the hypothalamus. TSH levels may be suppressed by excess free T3 (fT3) or free T4 (fT4) in the blood.

History

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]

Current status

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]

Interpretation

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]

Thyroid hormones

Total thyroxine

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:

Lower limitUpper limitUnit
4,[11] 5.5[12]11,[11] 12.3[12]μg/dL
60[11][13]140,[11] 160[13]nmol/L

Free thyroxine

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:

Patient typeLower limitUpper limitUnit
Normal adult0.7,[14] 0.8[12]1.4,[14] 1.5,[12] 1.8[15]ng/dL
9,[16][17] 10,[11] 12 [13]18,[16][17] 23[13]pmol/L
Infant 0–3 d2.0[14]5.0[14]ng/dL
26[17]65[17]pmol/L
Infant 3–30 d0.9[14]2.2[14]ng/dL
12[17]30[17]pmol/L
Child/Adolescent
31 d – 18 y		0.8[14]2.0[14]ng/dL
10[17]26[17]pmol/L
Pregnant0.5[14]1.0[14]ng/dL
6.5[17]13[17]pmol/L

Total triiodothyronine

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:

TestLower limitUpper limitUnit
Total triiodothyronine60,[12] 75[11]175,[11] 181[12]ng/dL
0.9,[16] 1.1[11]2.5,[16] 2.7[11]nmol/L

Free triiodothyronine

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:

Patient typeLower limitUpper limitUnit
Normal adult3.0[11]7.0[11]pg/mL
3.1[18]7.7[18]pmol/L
Children 2–16 y3.0[19]7.0[19]pg/mL
1.5[18]15.2[18]pmol/L

Carrier proteins

Thyroxine-binding globulin
Main article: Thyroxine-binding globulin

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:

Lower limitUpper limitUnit
12[12]30[12]mg/L

Thyroglobulin

Reference ranges:

Lower limitUpper limitUnit
1.5[11]30[11]pmol/L
1[11]20 [11]μg/L

Other binding hormones

Protein binding function

Thyroid hormone uptake

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. It is not directly related to triiodothyronine, despite the name T3 uptake.[2]

Reference ranges:

Patient typeLower limitUpper limitUnit
Females25[2]35[2] %
In pregnancy15[2]25[2] %
Males25[2]35[2] %

Other protein binding tests

Mixed parameters

Free thyroxine index

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]

Patient typeLower limitUpper limitUnit
Females1.8[2]5.0[2]
Males1.3[2]4.2[2]

Calculated and structure parameters
Reference ranges for thyroid's secretory capacity (SPINA-GT) and Jostel's TSH index (TSHI or JTI) compared to univariable reference ranges for thyrotropin (TSH) and free thyroxine (FT4), shown in the two-dimensional phase plane defined by serum concentrations of TSH and FT4.

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.[20][21][22][23]

Secretory capacity (GT)
Main article: Thyroid's secretory capacity

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.[24] GT is elevated in hyperthyroidism and reduced in hypothyroidism.[25]

GT is calculated with

or

: Dilution factor for T4 (reciprocal of apparent volume of distribution, 0.1 l−1)
: 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]

Lower limitUpper limitUnit
1.41[24]8.67[24]pmol/s

Sum activity of peripheral deiodinases (GD)
Main article: Sum activity of peripheral deiodinases

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

GD is obtained with

or

: Dilution factor for T3 (reciprocal of apparent volume of distribution, 0.026 L−1)
: 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]

Lower limitUpper limitUnit
20[24]40[24]nmol/s

TSH index
Main article: Jostel's TSH index

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

.

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

.[27]

ParameterLower limitUpper limitUnit
TSHI1.3[27]4.1[27]
sTSHI-2[27]2[27]

TTSI
Main article: Thyrotroph Thyroid Hormone Sensitivity Index

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.

Lower limitUpper limitUnit
100150

TFQI
Main article: Thyroid Feedback Quantile-based Index

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

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]

Lower limitUpper limitUnit
–0,74+0.74

Reconstructed set point

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]

Effects of drugs

Drugs can profoundly affect thyroid function tests. Listed below is a selection of important effects.

Effects of some drugs on Tests of Thyroid function[49][23][50]
CauseDrugEffect on hormone concentrationsEffect on structure parameters
Inhibited TSH secretionDopamine, L-DOPA, glucocorticoids, somatostatin↓T4; ↓T3; ↓TSH↔SPINA-GT; ↓JTI
Inhibited synthesis or release of thyroid hormoneIodine, lithium↓T4; ↓T3; ↑TSH↓SPINA-GT; ↔JTI
Inhibited conversion of T4 to T3 (Step-up hypodeiodination)Amiodarone, glucocorticoids, propranolol, propylthiouracil, radiographic contrast agents↓T3; ↑rT3; ↓, ↔, ↑T4 and fT4; ↔, ↑TSH↓SPINA-GD
Inhibited binding of T4/T3 to serum proteinsSalicylates, phenytoin, carbamazepine, furosemide, nonsteroidal anti-inflammatory agents, heparin (in vitro effect)↓T4; ↓T3; ↓fT4E, ↔, ↑fT4; ↔TSH↓T4/fT4 ratio
Stimulated metabolism of iodothyroninesPhenobarbital, phenytoin, carbamazepine, rifampicin↓T4; ↓fT4; ↔TSH
Inhibited absorption of ingested T4Aluminium hydroxide, ferrous sulfate, cholestyramine, colestipol, iron sucralfate, soybean preparations, kayexalate↓T4; ↓fT4; ↑TSH
Increase in concentration of T4-binding proteinsEstrogen, clofibrate, opiates (heroin, methadone), 5-fluorouracil, perphenazine↑T4; ↑T3; ↔fT4; ↔TSH↔SPINA-GT; ↔SPINA-GD; ↔JTI; ↑T4/fT4 ratio
Decrease in concentration of T4-binding proteinsAndrogens, glucocorticoids↓T4; ↓T3; ↔fT4; ↔TSH↔SPINA-GT; ↔SPINA-GD; ↔JTI; ↓T4/fT4 ratio

↓: 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

See also
Reference ranges for blood tests, sorted by mass and molar concentration, with thyroid function tests marked in purple boxes in left half of diagram.

References
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  15. Derived from molar values using molar mass of 776.87 g/mol
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  17. Derived from mass values using molar mass of 776.87 g/mol
  18. Derived from mass values using molar mass of 650.98 g/mol
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Further reading

CDC laboratory procedure manuals

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


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