Clinical Lab Handbook

Container/Tube: SST

Alternate Container: Red Top, Lithium Heparin-if ordering a screen or panel, lithium heparin is not acceptable for FT4

Specimen Volume: 5 mL of serum

Collection Instructions: Allow specimen to clot for 30 minutes. Spin specimen in centrifuge.

Chemiluminescence Immunoassay

An immunometric immunoassay technique is used,which involves the simultaneous reaction of TSH present in the sample with a biotinylated antibody (mouse monoclonal anti‑whole TSH) and a horseradish peroxidase (HRP)-labeled antibody conjugate (mouse monoclonal anti-TSH β-subunit). The antigen-antibody complex is captured by streptavidin on the wells. Unbound materials are removed by washing.

The bound HRP conjugate is measured by a luminescent reaction. A reagent containing luminogenic substrates (a luminol derivative and a peracid salt) and an electron transfer agent, is added to the wells. The HRP in the bound conjugate catalyzes the oxidation of the luminol derivative, producing light. The electron transfer agent (a substituted acetanilide) increases the level of light produced and prolongs its emission. The light signals are read by the system. The amount of HRP conjugate bound is directly proportional to the concentration of TSH present.

TSH secretion by the anterior pituitary is controlled by thyrotropin releasing hormone, a tripeptide produced by the hypothalamus. TSH stimulates the production of thyroxine (T4) and triiodothyronine (T3) by the thyroid gland. The circulating free fractions of T4 and T3 in turn regulate the secretion of TSH by a negative feedback mechanism at the pituitary and possibly the hypothalamus. The diagnosis of overt hypothyroidism by the finding of a low total T4 or free T4 concentration is readily confirmed by a raised TSH concentration.

Measurement of low or undetectable TSH concentrations may assist the diagnosis of hyperthyroidism, where concentrations of T4 and T3 are elevated and TSH secretion is suppressed. TSH tests with high levels of precision and functional sensitivity claims of 0.01–0.02 mIU/L have been termed “third generation” tests. These have the advantage of discriminating between the concentrations of TSH observed in thyrotoxicosis, compared with the low, but detectable, concentrations that occur in subclinical hyperthyroidism.

0.465-4.680 mIU/mL

Monday through Sunday

84443-TSH

84439-FT4 (if reflex or screen is ordered only)