Overview
Thyroid function tests assess the hypothalamic-pituitary-thyroid (HPT) axis through TSH and free thyroid hormones. Many institutional pathways use TSH as first-line screening, adding free T4 for confirmation and severity, and free T3 selectively when clinically indicated.
Immunoassay methods, binding protein changes, pregnancy, non-thyroidal illness, biotin, and heterophile antibodies can distort results, method identity and population context are bench prerequisites.
Classification
| Axis | Key markers | Bench note |
|---|---|---|
| Thyroid (HPT) | TSH, free T4, free T3 | TSH often first-line; free hormones for confirmation |
| Adrenal (HPA) | Cortisol, ACTH | Diurnal timing; ACTH preanalytics critical |
| Pancreatic/endocrine | Insulin, C-peptide | C-peptide reflects endogenous production |
| Mineral | PTH, calcium, 25-OH D | Interpret PTH with calcium; verify D method/limits |
| Immunoassay pitfalls | Binding, biotin, heterophile | Method and population context matter |
Morphologic Features
Chemistry marker reference, morphologic cell identification is hematology, not endocrine biomarker chemistry framing.
Laboratory Characteristics
- Confirm method identity and units for endocrine hormone assays
- Compare to method-specific reference intervals (verify locally)
- Document collection time, especially cortisol diurnal and hypoglycemia pairing
- Watch biotin, heterophile antibodies, and hook effect per method SOP
Reference Intervals
Reference intervals and decision limits for endocrine biomarkers are method- and institution-dependent. Verify institutional and method-specific intervals, do not invent absolute universal cutoffs as dogma.
| Parameter | Reporting note | Action |
|---|---|---|
| TSH | Method-specific URL; assay generation matters | Verify local RI; trimester/age frames if used |
| Free T4 / free T3 | Binding-protein and method effects | Pair with TSH; verify equilibrium vs analog methods |
| Cortisol | Diurnal; serum/plasma vs saliva/urine per local use | Document collection time; verify local cutoffs |
| ACTH | Unstable peptide; cold handling | Verify institutional collection SOP |
| Insulin / C-peptide | Immunoassay; hypoglycemia timing with glucose | Verify local hypoglycemia workup framing |
| PTH | Intact PTH assays; pair with calcium | Verify assay generation and calcium context |
| 25-OH vitamin D | Status marker; immunoassay vs LC-MS/MS | Verify institutional decision limits |
Clinical and Laboratory Significance
Thyroid panels guide laboratory classification of hyperthyroid, hypothyroid, and subclinical patterns, always paired with clinical context and verified intervals.
Panel composition and reflex rules vary by institution; the laboratory must document assay method, units, and any trimester- or age-specific reference framing when used.
Differential Considerations
Primary vs secondary axis patterns vs binding/method effects vs preanalytical artifact. Chemistry suggests; clinical correlation decides.
Comparison Tables
Endocrine biomarker laboratory framing
| Tool | Typical lab clue | Caveat |
|---|---|---|
| TSH | Primary thyroid screening in many pathways | Assay generation; pregnancy/illness effects |
| Free T4 | Thyroid hormone confirmation/severity | Binding proteins; method differences |
| Free T3 | Selective use (e.g., T3 thyrotoxicosis suspicion) | Not routine in all pathways |
| Cortisol + ACTH | Adrenal axis laboratory framing | Timing and preanalytics dominate |
| Insulin vs C-peptide | Hypoglycemia laboratory differentiation | Exogenous insulin may not match C-peptide |
| PTH + calcium | Mineral metabolism pattern | Primary vs secondary patterns, lab only |
| 25-OH D | Vitamin D status | 1,25-(OH)2D is a different clinical question |
Classification Frameworks
Endocrine laboratory interpretive frameworks may guide hormone evaluation, always verify current institutional adoption and guideline version; do not invent absolute universal cutoffs as dogma. LabPedia provides laboratory framing only, no treatment recommendations.
Laboratory Notes
- Never invent absolute universal diagnostic cutoffs as dogma
- Verify institutional and method-specific reference intervals
- Pair companion markers per axis feedback logic
References
Authoritative textbooks, guidelines, and reviews supporting this reference entry. Verify reference intervals, critical limits, and reflex criteria against institutional protocols and current guideline editions.
Textbooks
- Rifai N, Horvath AR, Wittwer CT, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 8th ed. Elsevier; 2022.
- McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021.
- Bishop ML, Fody EP, Schoeff LE. Clinical Chemistry: Principles, Techniques, and Correlations. 9th ed. Jones & Bartlett; 2022.
CLSI and Professional Guidelines
- Endocrine laboratory testing and interpretation guidance (verify current institutional protocol and any adopted endocrine testing guidance for thyroid, adrenal, and mineral metabolism pathways).
- Clinical and Laboratory Standards Institute. Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory; Approved Standard. CLSI document EP28-A3c. CLSI; 2010.
Frequently Asked Questions
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Thyroid Function Tests . Chemistry is a Markers and Analytes entry in the LabPedia Medical Laboratory Library, written for Medical Laboratory Science reference lookup.
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