Thyroid Function Cascade, Serum
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
The Thyroid Function Cascade, Serum utilizes a cascaded testing procedure to efficiently evaluate and monitor functional thyroid status. When ordered, physicians automatically obtain a series of tests that will rapidly provide comprehensive thyroid results for their patients. This test will significantly reduce the cost of care for patients by eliminating downstream costs associated with expensive redraws, repeat orders, return office visits, extended hospital stays, and physician time.
The cascade begins with sensitive-thyroid–stimulating hormone (s-TSH), a highly effective screening assay. In patients with an intact pituitary-thyroid axis, s-TSH provides a physiologic indicator of the functional level of thyroid hormone activity. Increased s-TSH indicates inadequate thyroid hormone, and suppressed s-TSH indicates excess thyroid hormone.
Transient s-TSH abnormalities may be found in seriously ill, hospitalized patients, so this is not the ideal setting to assess thyroid function. However, even in these patients, s-TSH works better than total thyroxine (an alternative screening test).
The majority of these screens will be normal and no further testing will be necessary. However, when the s-TSH result is abnormal, appropriate follow-up tests will automatically be performed.
If s-TSH is <0.3 mIU/L or >5.0 mIU/L, free thyroxine (FT4) is performed. The supplemental measurement of FT4 in patients with abnormal s-TSH measurements allows one to better asses the severity of the changes.
Serum triiodothyronine (T3) levels often are depressed in sick and hospitalized patients, caused in part by the biochemical shift to the production of reverse T3. Therefore, T3 generally is not a reliable predictor of hypothyroidism. However, in a small subset of hyperthyroid patients, hyperthyroidism may be caused by overproduction of T3 (T3 toxicosis). To help diagnose and monitor this subgroup, T3 is measured on all specimens with suppressed s-TSH and normal FT4 concentrations.
Detectable concentrations of antithyroperoxidase (anti-TPO) antibodies are observed in patients with autoimmune thyroiditis and may cause the destruction of thyroid tissue, resulting in the eventual hypothyroidism. Anti-TPO antibodies are measured in all specimens with elevated s-TSH concentrations.
See Thyroid Function Ordering Algorithm in Special Instructions.
Screening for a diagnosis of thyroid disease
Thyroid function assessment using a cost- and time-efficient testing algorithm
See individual unit codes for additional information.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Two important patient groups may require alternate thyroid function testing strategies: patients with pituitary disease and patients with neuropsychiatric disorders. These patients may require endocrinologic consultations, because the sensitive-thyroid stimulating hormone measurement may be misleading and additional tests probably are necessary.
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.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
> or =12 months: 0.3-5.0 mIU/L
Reference values have not been established for patients that are <12 months of age.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Feldt-Rasmussen U: Analytical and clinical performance goals for testing autoantibodies to thyroperoxidase, thyroglobulin and thyrotropin receptor. Clin Chem 1996;42:160-163
2. Fatourechi V, Lankarani M, Schryver PG, et al: Factors influencing clinical decisions to initiate thyroxine therapy for patients with mildly increased serum thyrotropin (5.1-10.0 mIU/L). Mayo Clin Proc 2003;78:554-560
3. Wilson J, Foster D, Kronenburg H, et al: Williams Textbook of Endocrinology. Ninth edition, WB Saunders Company, 1998