Thyrotrophin releasing hormone (TRH), a tripeptide, is secreted from the hypothalamus. It runs through the hypothalamo-pituitary portal vessels to the anterior pituitary gland to stimulate the production of thyroid stimulating hormone (TSH). TSH via the bloodstream reaches the follicular cells of the thyroid gland. L-thyroxine (T4) and triiodothyroxine (T3) are synthesised and 99per cent of them are bound to Thyroid binding globulins (TBG). Only unbound T3 and T4 are active. T3 and T4 acts on nuclear receptors to exert the functions described above. T3 is five times more active than T4. Most T3 are formed from conversion of T4 from the peripheries. T3 and T4 forms negative feedback loop to the thyroid and hypothalamus inhibiting the production and release of TSH and TRH. {{10 'Murray Longmore', 'Ian Wilkinson', 'Tom Turmezei', 'Chee Kay Cheung' 2007; 7 Kumar,Parveen J. 2010}}
The imbalance of these hormones can cause a number of endocrine conditions. Diagnosis of thyroid diseases depends largely on serum TSH and thyroid hormone levels. Thyroid Function Test (TFTs) is performed via immunoassays to determine the level of Thyroid Stimulating Hormone (TSH), free T3 and free T4. Thyroid hormone does have the circadian rhythm, but the effect of this is ignorable. {{7 Kumar,Parveen J. 2010}}
These Hormone assays are performed to confirm the diagnosis of thyroid disease as well as to investigate the cause of the disease. In some cases it is possible to determine the causes. In some borderline cases, a series of other tests may be performed. They are thyroid autoantibody tests, TSH receptor antibody, serum thyroglobulin, ultrasound, and isotope scan.{{10 'Murray Longmore', 'Ian Wilkinson', 'Tom Turmezei', 'Chee Kay Cheung' 2007}} These tests in combination with clinical history and physical examination aids the diagnosis overall.
There has been improvement in technology of performing immunoassay for last two decades, and serum TSH is the most sensitive method for thyroid function assessment. Serum TSH is shown to have sensitivitiy from 89 to 95% and specificity at range of 90 to 96% for obvious thyroid dysfunction.{{22 Davies T.F, Spencer C.A., Demers L.M. 2003}}{{11 'Graham H Beastall', 'Geoffrey J Beckett', 'Jayne Franklyn', 'William D Fraser', 'Janis Hickey', 'Rhys John', 'Pat Kendall-Taylor', 'betty nevans', 'Mark Vanderpump' 2006}}
Due to high sensitivity and specificity of the test, measuring serum TSH alone is widely performed in clinical practice. This is due to cost-effectivity. However, there are some situations where TSH alone may not give enough information to deduce the cause of the thyroid disease. In this case, TSH and T4 are both measured at the same time. These cases include: assay interference, unstable thyroid axis, and organ resistance. {{14 'Anthony D. Toft', 'Geoffery J. Beckett' 2005}}
TRH administration is rarely performed clinically. Only time it is performed is when hypothalamopituitary are guessed by the practitioner. Administration of TRH may be performed in patients with suspected non-thyroidal illness. {{20 Shivaraj G, Prakash BD, Sonal V, Shruthi K, Vinayak H, Avinash M 2009}}
Diagnostic accuracy
1)Specificity
TSH is present in different epitopes. IRMA technique can detect different forms of TSH and consequently increasing specificity. This is achieved by the action of TSH monoclonal antibody preventing glycoprotein hormone action on TSH. {{22 Davies T.F, Spencer C.A., Demers L.M. 2003}}{{11 'Graham H Beastall', 'Geoffrey J Beckett', 'Jayne Franklyn', 'William D Fraser', 'Janis Hickey', 'Rhys John', 'Pat Kendall-Taylor', 'betty nevans', 'Mark Vanderpump' 2006}}
2)Sensitivity
Functional sensitivity is lowest concentration of the assay when 10 percents of upper and lower limit distribution is removed. IRMA used these days displays this value less than 0.02mLU/L. This is essential to achieve full range sensitivity for detecting thyroid diseases (i.e. distinguishing hyperthyroidism and hypothyroidism){{11 'Graham H Beastall', 'Geoffrey J Beckett', 'Jayne Franklyn', 'William D Fraser', 'Janis Hickey', 'Rhys John', 'Pat Kendall-Taylor', 'betty nevans', 'Mark Vanderpump' 2006}}
Quality assurance programme, both internal control and external control are held regularly to ensure the sensitivity and specificity of the assays.
B. FT4 and FT3 Estimate Tests
Majority of thyroid hormones are bound to protein such as TBG, and only 0.05 and 0.2 per cent of T4 and T3 are free respectively. It is the free thyroid hormones which are active in peripheries. Free T3 and T4 are dectected by equilibrium dialysis or ultrafiltration which involves separation of the hormones from TBG. However, these methods are expensive and there is high risk of failure. They are therefore not performed in reference laboratories. Although the method is not accurate, it is considered to be superior to measuring total T3 and T4.{{22 Davies T.F, Spencer C.A., Demers L.M. 2003}} Also, TSH and T4 relationship is considered superior to T4 concentration. {{22 Davies T.F, Spencer C.A., Demers L.M. 2003}}
Small or medium sized laboratories use index methods to estimate the free T3 or T4 concentration. One example of this is ‘one-test free hormone estimates’{{14 'Anthony D. Toft', 'Geoffery J. Beckett' 2005}}
