Clinical use
TSH is secreted by the pituitary gland. It is composed of two subunits called α and ß. Other hormones such as LH, FSH and HCG have virtually identical α subunits, but the ß subunits are different. These differences are exploited by immunoassay.
The main function of TSH is to regulate the release of T4 and T3 from the thyroid. High levels of these hormones exert a negative feedback effect on the pituitary, inhibiting the release of TSH. Low levels of T4 or T3 act upon the hypothalamus causing it to release thyroid releasing hormone (TRH). This stimulates the pituitary to produce TSH, which in turn causes the release of T4 and T3 from the thyroid. In the normal individual there is a dynamic equilibrium established between the hormones which maintain normality.
TSH is the “Front Line” assay for Thyroid Function Testing in this department. In the absence of a previous clinical history of Thyroid Disease, only the TSH will be measured. Further investigation may be indicated by the TSH result obtained.
Reference range
0.27 – 4.2m U/l
Specimen requirements
Serum separating tube (SST)
Please see the blood bottles page for additional information on blood bottles.
Please see the form and labelling page for additional information on sample identification requirements.
Limitations and restrictions
Patients exposed to animals or animal serum products can produce heterophilic antibodies, these can interfere with this assay and produce anomalous results.
This assay has not been validated for testing samples from newborns.
Evidence suggests that patients undergoing fluorescein dye angiography can retain small amounts of fluorescein in the body for up to 48 to 72 hours post-treatment. In the cases of patients with renal insufficiency, retention could be much longer. Samples containing fluorescein can produce falsely depressed values when tested with the ADVIA Centaur TSH3 Ultra assay. With fluorescein interference, observed TSH3 Ultra values can be as low as <0.01 mIU/L. These falsely low values are important when monitoring for thyroid cancer as the physician may determine that adequate suppression of TSH has been obtained. Samples should be resubmitted post fluorescein clearance to ensure there is no interference with TSH3 Ultra test results.
Turnaround time
6 hours
Analysing laboratory
- Biochemistry Lab, James Cook University Hospital, Marton Road, TS4 3BW
- Biochemistry Lab, Friarage Hospital, Northallerton, DL6 1JG