Thyroid Function Tests - Best Thyroid Test

Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid.

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.

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 hormones

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Thyroid-stimulating hormone

Thyroid-stimulating hormone (TSH, thyrotropin) is generally increased in hypothyroidism and decreased in hyperthyroidism.

Its measurement is the most sensitive test for thyroid hormone function. 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.

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Total thyroxine is rarely measured, having been largely superseded by free thyroxine tests.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. It is usually slightly elevated in pregnancy secondary to increased levels of thyroid binding globulin (TBG).

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

Free thyroxine (fT₄) is generally elevated in hyperthyroidism and decreased in hypothyroidism.

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Total triiodothyronine (Total T₃) is rarely measured, having been largely superseded by free T3 tests.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.

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

Free triiodothyronine (fT₃) is generally elevated in hyperthyroidism and decreased in hypothyroidism.

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:

Carrier proteins

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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:

Thyroglobulin

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Reference ranges:

Other binding hormones

• Transthyretin (prealbumin)
• Albumin

Protein binding function

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Thyroid hormone uptake

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. Unsaturated TBG increases with decreased levels of thyroid hormones. It is not directly related to triiodothyronine, despite the name T_{3 uptake}.

Reference ranges:

Other protein binding tests

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

Mixed parameters

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Free thyroxine index

The Free Thyroxine Index (FTI or T7) is obtained by multiplying the total T₄ with T_uptake. FTI is considered to be a more reliable indicator of thyroid status in the presence of abnormalities in plasma protein binding. 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.

Structure parameters

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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

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

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^âˆ'1)
${\displaystyle \beta _{T}}$: Clearance exponent for T4 (1.1e-6 sec^âˆ'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)

Sum activity of peripheral deiodinases (GD)

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

GD 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^âˆ'1)
${\displaystyle \beta _{31}}$: Clearance exponent for T3 (8e-6 sec^âˆ'1)
K_M1: Dissociation constant of type-1-deiodinase (5e-7 mol/l)
K₃₀: Dissociation constant T3-TBG (2e9 l/mol)

TSH index

Jostel's TSH index (TSHI) helps to determine thyrotropic function of anterior pituitary on a quantitative level. It is reduced in thyrotropic insufficiency and in certain cases of non-thyroidal illness syndrome.

It is calculated with

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

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

${\displaystyle sTSHI=(TSHI-2.7)/0.676}$.

Effect of drugs

Listed below are some of the effects of drugs on thyroid function.

â†": reduced serum concentration; â†': increased serum concentration; â†": non change; TSH: Thyroid-stimulating hormone; T₃: Total triiodothyronine; T₄: Total thyroxine; fT₄: Free thyroxine; fT₃: Free triiodothyronine; rT₃: Reverse triiodothyronine

TTSI

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. Somewhat similar to the TSH Index it is calculated from equilibrium values for TSH and FT4, however with a different equation.

See also

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• Reference ranges for blood tests#Thyroid hormones

References

Further reading

• American Thyroid Association: Thyroid Function Tests. Posted on June 4, 2012, seen on January 9, 2013.
• Thyroid function panel - Lab Tests Online

External links

• SPINA Thyr: Open source software for calculating GT and GD