TSH (Thyroid Stimulating Hormone), serum

Clinical background:

Thyroid stimulating hormone (TSH) is a gIycoprotein produced by the anterior pituitary. The biologically active TSH molecule consists of two subunits designated as alpha and beta chains. The alpha chains are nearly identical in form to other gIycoprotein hormones produced by the anterior pituitary: HCG, hFSH, and LH. The beta chain heterogeneity differentiates these hormones from each other. TSH is responsible for the control of synthesis and secretion of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). TSH secretion is regulated by thyrotropin releasing hormone (TRH), which is secreted by the hypothalamus, and by direct action of thyroid hormones at the pituitary gland. Rising levels of T4 and T3 act as a negative feedback mechanism of the anterior pituitary to inhibit the stimulatory effects of TRH.

The measurement of TSH is one of the most important tools in the diagnosis of thyroid disorders. An increase in serum TSH levels is an early indicator of thyroid insufficiency and in association with a reduction of serum free T4, can be indicative of primary hypothyroidism. It may also be used in the differential diagnosis of secondary and tertiary (hypothalamic) hypothyroidism from primary hypothyroidism. Low or undetectable (<0.05mu/L) concentrations of TSH, when accompanied by an elevated free T4 and/or free T3 level is an important indicator in the diagnosis of hyperthyroidism. The measurement of TSH is also used as a monitoring tool for those patients on thyroid replacement therapy (T4 and/or T3), with elevated TSH suggesting under-dosage or non compliance and low TSH suggestive of over treatment.

Specimen container paediatric:

Serum (SST or plain tube)

Specimen container adult:

Serum (SST or plain tube)

Minimum volume paediatric:

0.5 mL 

Minimum volume adult:

1 mL 

Sample stability:

Unseparated:

7 days

Separated:

1 day at 20-25C, 7 days at 4-8°C, 1 month at -20°C

Transport requirements:

Ambient

Interpretation:

Low TSH with raised FT4 (and/or FT3) is consistent with primary hyperthyroidisim. Raised TSH with low FT4 is consistent with hypothyroidism. ‘Sub-clinical’ or borderline hypo/hyperthyroidism can be associated with a FT4 within the reference range but abnormal TSH. Low FT4 with an inappropriately low or normal TSH can be associated with secondary hypothyroidism. It is important to note that non-thyroidal illness can cause abnormal thyroid function tests.

Reference ranges:

Other info:

Lithium heparin plasma samples also acceptable