Summary & Overview
CPT 84439: Free Thyroxine (Free T4) Assay
CPT code 84439 identifies a laboratory test for free thyroxine (free T4), a key measure in thyroid function assessment used to diagnose and manage hypo- and hyperthyroid conditions. Free T4 testing is widely performed across outpatient and hospital laboratory settings and informs clinical decisions about thyroid hormone status and therapy. Nationally, this assay is a routine component of endocrine and primary care workups and influences medication management, monitoring of thyroid disease, and evaluation of abnormal thyroid function studies.
Key payers addressed include Aetna, Blue Cross Blue Shield, Cigna Health, UnitedHealthcare, and Medicare. Readers will find a concise clinical context for the test, connections to commonly reported ICD-10 diagnoses associated with thyroid disorders, and links to related laboratory codes such as total thyroxine and thyroid hormone uptake. The publication summarizes typical sites of service and common clinical indications, outlines where 84439 fits within laboratory service lines, and identifies associated procedural neighbors used in thyroid panels.
This resource provides operational clarity for coding teams, billing staff, and clinical administrators seeking a national-level reference for the code’s clinical role, common diagnostic associations, and related laboratory services. Data not available in the input is noted where applicable.
Billing Code Overview
CPT code 84439 describes a laboratory assay measuring free thyroxine (free T4), typically performed on patient serum. The service involves analytic measurement by a clinical laboratory analyst to assess circulating unbound T4 levels, which reflect thyroid hormone available to tissues.
Service type: Laboratory diagnostic test
Typical site of service: Clinical laboratory or hospital outpatient laboratory
Clinical & Coding Specifications
Clinical Context
A 45-year-old woman presents to her primary care clinician with fatigue, weight gain, cold intolerance, and constipation. The clinician obtains a serum specimen and orders a free thyroxine test to evaluate thyroid status. In the clinical workflow, the phlebotomist collects blood in the outpatient laboratory; the Clinical Medical Laboratory (CLIA-certified) processes the sample and the laboratory analyst performs the 84439 assay for free T4. Results are returned to the Family Medicine or Internal Medicine clinician who correlates free T4 with thyroid-stimulating hormone and other thyroid studies to confirm or monitor diagnoses such as E03.9 (Hypothyroidism), E06.3 (Autoimmune thyroiditis), or E05.90 (Thyrotoxicosis). Follow-up may include medication adjustment, repeat laboratory monitoring, or referral to endocrine specialty care.
Coding Specifications
| Modifier | Description | When to Use |
|---|---|---|
26 | Professional component | Use when billing only the physician interpretation/professional component for a lab test when separated from the technical component. |