Interpretive Handbook

Thyroid-stimulating hormone (TSH) is a glycoprotein hormone that has 2 subunits. The alpha-subunit is similar to those of follicle-stimulating hormone, human chorionic gonadotropin, and luteinizing hormone. The beta-subunit is different from those of the other glycoprotein hormones and confers its biochemical specificity.

TSH is synthesized and secreted by the anterior pituitary in response to a negative feedback mechanism involving concentrations of free triiodothyronine and free thyroxine. Additionally, the hypothalamic tripeptide, thyrotropin-releasing hormone, directly stimulates TSH production.

TSH interacts with specific cell receptors on the thyroid cell surface and gives rise to 2 main actions. First, it stimulates cell reproduction and hypertrophy. Second, it stimulates the thyroid gland to synthesize and secrete triiodothyronine and thyroxine.

In primary hypothyroidism, TSH levels will be elevated. In primary hyperthyroidism, TSH levels will be low.

See Thyroid Function Ordering Algorithm in Special Instructions.

Monitoring patients on thyroid replacement therapy

Confirmation of thyroid-stimulating hormone (TSH) suppression in thyroid cancer patients on thyroxine therapy

Prediction of thyrotropin-releasing hormone-stimulated TSH response

An aid in the diagnosis of primary hyperthyroidism

Differential diagnosis of hypothyroidism

The ability to quantitate circulating levels of thyroid-stimulating hormone (TSH) is important in evaluating thyroid function. It is especially useful in the differential diagnosis of primary (thyroid) from secondary (pituitary) and tertiary (hypothalamus) hypothyroidism. In primary hypothyroidism, TSH levels are significantly elevated, while in secondary and tertiary hypothyroidism, TSH levels are low or normal. Concentrations of 5.1 mIU/L to 7.0 mIU/L are considered borderline hypothyroid.

For primary hyperthyroidism, a high-sensitivity TSH assay is required to detect the decrease in TSH with sufficient diagnostic accuracy. This assay is an enhanced version of the sensitive TSH assay, and has a sensitivity down to 0.002 mIU/L.

Thyrotropin-releasing hormone (TRH) stimulation differentiates all types of hypothyroidism by observing the change in patient TSH levels in response to TRH. Typically, the TSH response to TRH stimulation is exaggerated in cases of primary hypothyroidism, absent in secondary hypothyroidism, and delayed in tertiary hypothyroidism. Most individuals with primary hyperthyroidism have TSH suppression and do not respond to TRH stimulation test with an increase in TSH over their basal value. Based on a study at Mayo Clinic, if the s-TSH result is < or =0.1 mIU/L, there is only about a 1% chance the patient will respond to a TRH stimulation test. Patients with s-TSH values between 0.1 to 0.4 mIU/L sometimes respond, whereas patients with values > or =0.4 mIU/L will have a complete or partial response to TRH.

Sick, hospitalized patients may have falsely low or transiently elevated thyroid stimulating hormone.

Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedure, may have circulating antianimal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.

> or =12 months: 0.3-5.0 mIU/L

Reference values have not been established for patients that are less than 12 months of age.

1. Fatourechi V, Lankarani M, Schryver P, et al: Factors influencing clinical decisions to initiate thyroxine therapy for patients with mildly increased serum thyrotropin (5.1-10.0 mIU/L). (Journal Article) Mayo Clin Proc 2003 May;78(5):554-560

2. Klee G, Hay I: Biochemical testing of thyroid function. Review. Endocrinol Metab Clin North Am 1997 Dec:26(4):763-775

3. Wilson J, Foster D, Kronenburg MD H, et al: Textbook of Endocrinology. Ninth edition, WB Saunders Company, 1998