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HelpTest ID: THSCM Thyroid Function Cascade, Serum
Reporting Name
Thyroid Function Cascade, SUseful For
Screening for a diagnosis of thyroid disease
Clinical Information
This test utilizes a cascaded testing procedure to efficiently evaluate and monitor functional thyroid status.
The cascade begins with thyroid-stimulating hormone (TSH) as a screening assay. In patients with an intact pituitary-thyroid axis, TSH provides a physiologic indicator of the functional level of thyroid hormone activity. Increased TSH indicates inadequate thyroid hormone, and suppressed TSH indicates excess thyroid hormone.
Transient 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, TSH works better than total thyroxine (an alternative screening test).
When TSH is normal, no additional testing will be necessary. However, when the TSH result is abnormal, appropriate follow-up tests will automatically be performed.
If TSH is below 0.3 mIU/L or above 4.2 mIU/L, free thyroxine (FT4) is performed. The supplemental measurement of FT4 in patients with abnormal TSH measurements allows one to better assess 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 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, eventually resulting in hypothyroidism. Anti-TPO antibodies are measured in all specimens with elevated TSH concentrations.
See Thyroid Function Ordering Algorithm in Special Instructions.
Interpretation
In primary hypothyroidism, thyroid-stimulating hormone (TSH) levels will be elevated. In primary hyperthyroidism, TSH levels will be low.
The ability to quantitate circulating levels of 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.
Elevated or low TSH in the context of normal free thyroxine is often referred to as subclinical hypo- or hyperthyroidism, respectively.
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.
Sick, hospitalized patients may have falsely low or transiently elevated TSH.
Testing Algorithm
If thyroid-stimulating hormone (TSH) is <0.3 mIU/L, then free T4 (FT4) is performed at an additional charge.
If FT4 is normal and the TSH is <0.1 mIU/L, then triiodothyronine (T3) is performed at an additional charge.
If TSH is >4.2 mIU/L, then FT4 and thyroperoxidase antibodies are performed at an additional charge.
See Thyroid Function Ordering Algorithm in Special Instructions.
Profile Information
| Test ID | Reporting Name | Available Separately | Always Performed |
|---|---|---|---|
| STSHC | TSH, Sensitive, S | Yes, (order STSH) | Yes |
Reflex Tests
| Test ID | Reporting Name | Available Separately | Always Performed |
|---|---|---|---|
| FRT4C | T4 (Thyroxine), Free, S | Yes, (order FRT4) | No |
| TPOC | Thyroperoxidase Ab, S | Yes, (order TPO) | No |
| T3C | T3 (Triiodothyronine), Total, S | Yes, (order T3) | No |
Special Instructions
Analytic Time
1 dayDay(s) and Time(s) Performed
Monday through Sunday; Continuously
Clinical Reference
1. Package insert: Roche TSH Reagent, Roche Diagnostics, Indianapolis, IN, 2010-08, V2
2. 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). Mayo Clin Proc 2003 May;78(5):554-560
3. Wilson JD, Foster D, Kronenburg HM, et al: Williams Textbook of Endocrinology. Ninth edition. WB Saunders Company, 1998
4. Melmed S, Polonsky KS, Larsen PR, et al: Williams Textbook of Endocrinology. 12th edition. Elsevier Saunders Company, 2011, pp 348-414
5. Heil W, Ehrhardt V: Reference Intervals for Adults and Children 2008. Ninth edition. Roche Diagnostics Ltd, Rotkreuz, Switzerland July 2009, V9.1
Method Name
Electrochemiluminescent Immunoassay
Specimen Type
SerumSpecimen Required
Patient Preparation:
1. For 12 hours before specimen collection do not take multivitamins or dietary supplements containing biotin (vitamin B7), which is commonly found in hair, skin, and nail supplements and multivitamins.
2. In patients receiving therapy with high biotin doses (ie, >5 mg/day), no specimen should be taken until at least 8 hours after the last biotin administration.
Container/Tube:
Preferred: Serum gel
Acceptable: Red top
Specimen Volume: 1.5 mL
Collection Instructions:
1. Serum gel tubes should be centrifuged within 2 hours of collection.
2. Red-top tubes should be centrifuged and aliquoted within 2 hours of collection.
Specimen Minimum Volume
1 mL
Specimen Stability Information
| Specimen Type | Temperature | Time | Special Container |
|---|---|---|---|
| Serum | Refrigerated (preferred) | 7 days | |
| Frozen | 30 days | ||
| Ambient | 72 hours |
Reference Values
0-5 days: 0.7-15.2 mIU/L
6 days-2 months: 0.7-11.0 mIU/L
3-11 months: 0.7-8.4 mIU/L
1-5 years: 0.7-6.0 mIU/L
6-10 years: 0.6-4.8 mIU/L
11-19 years: 0.5-4.3 mIU/L
≥20 years: 0.3-4.2 mIU/L
Test Classification
This test has been cleared, approved or is exempt by the U.S. Food and Drug Administration and is used per manufacturer's instructions. Performance characteristics were verified by Mayo Clinic in a manner consistent with CLIA requirements.CPT Code Information
84443-Thyroid-stimulating hormone-sensitive (s-TSH)
84439-T4 (thyroxine), free (if appropriate)
84480-T3 (triiodothyronine), total (if appropriate)
86376-Thyroperoxidase (TPO) antibodies (if appropriate)
LOINC Code Information
| Test ID | Test Order Name | Order LOINC Value |
|---|---|---|
| THSCM | Thyroid Function Cascade, S | 11579-0 |
| Result ID | Test Result Name | Result LOINC Value |
|---|---|---|
| STSHC | TSH, Sensitive, S | 11579-0 |
mml-endo-thyroid