Test Information

The TSH assay exhibits performance consistent with the definition of third generation. TSH is most often used for the evaluation of the thyroid axis, proper understanding of its utility and limitations is thus clinically important.

The principal clinical use for TSH measurement is for the assessment of thyroid status. In patients with intact hypothalamic-pituitary function, TSH is measured to: 1) exclude hypothyroidism (elevated levels of TSH) or hyperthyroidism (depressed or non-detectable levels of TSH); 2) monitor T4 replacement treatment in primary hypothyroidism or antithyroid treatment in hyperthyroidism; 3) follow T4 suppression of the trophic influence of TSH in cold nodules and non-toxic goiter; and 4) assess the response to TRH stimulation testing.

TSH concentrations follow a diurnal rhythm: it typically peaks around midnight and nadir around mid-day. Reference intervals are generally obtained from subjects tested in the daytime, close to nadir than peak, therefore, when evaluating patients serial TSH concentrations, differences in sample collection time should be considered.

TSH Variability: TSH has moderate intra-individual variability and marked inter-individual variability. Since the intra-individual variation is considerably less, when comparing a specific patients current TSH level a better approach is to compare with any past level than comparing the patients current TSH level to the reference interval. A difference of 0.7 mIU/L or greater is considered significant when evaluating a patients serial TSH values.

Method Dependency: TSH methods do not always yield the same result. As much as a 10% difference between results may be generated from different TSH methods. Therefore, the same method should be used when monitoring TSH concentration over time. Similarly TSH reference intervals are also method-dependent and are appropriately applied only to patient results generated from the same method.

Pregnancy: It is reasonable to consider serum TSH measurement for pregnant women or women planning to become pregnant with a family history of thyroid disease, prior thyroid dysfunction, symptoms or physical findings suggestive of hypo- or hyperthyroidism, an abnormal thyroid gland on examination, type 1 diabetes mellitus, or a personal history of autoimmune disorder. Suggested upper limit for the TSH reference range for pregnant women and preconception is: first trimester <2.5 IU/mL, and 3.0 IU/mL in the second and third trimesters (The American Thyroid Association (ATA), 2011).