Thyroxine, Free (Free T4)
CPT Code(s):
84439
Turnaround Time:
4 hours
Specimen Requirements
Preferred Specimen:
Serum
Minimum Volume:
1.0 mL
Transport Container:
Serum Separator Tube (SST) or Red Top Tube
Transport Temperature:
Room Temperature
Collection Instructions:
Allow blood to clot completely prior to centrifugation. Separate serum from cells within 45 minutes of collection.
Rejection Criteria:
Citrate or Fluoride plasma specimen; gross hemolysis; lipemia; improper labeling
Stability Criteria
Room Temperature:
5 days
Refrigerated:
7 days
Frozen:
30 days
Clinical Significance
Free T4 holds specific clinical significance and diagnostic use in assessing thyroid function and diagnosing thyroid disorders. Thyroxine (T4) is a thyroid hormone that plays a crucial role in regulating metabolism, growth, and development in the body. Free T4 measures the amount of unbound or active T4 in the blood, providing important information about thyroid function.
The specific clinical significance and diagnostic use of Free T4 include:
1. Assessment of Thyroid Function: Free T4 is used to evaluate thyroid function and assess the production and activity of T4 hormone. It helps differentiate between primary, secondary, and tertiary hypothyroidism and hyperthyroidism, providing insights into the underlying cause of thyroid dysfunction. Abnormal Free T4 levels, along with other thyroid function tests, aid in diagnosing and monitoring thyroid disorders.
2. Diagnosis of Hypothyroidism: Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormones, resulting in symptoms such as fatigue, weight gain, cold intolerance, and depression. Free T4 plays a key role in diagnosing hypothyroidism, as low levels of Free T4 indicate reduced thyroid hormone activity. It helps healthcare providers assess the severity of hypothyroidism and guide appropriate treatment strategies.
3. Monitoring Thyroid Replacement Therapy: Free T4 is used to monitor the effectiveness of thyroid hormone replacement therapy in individuals with hypothyroidism. Regular monitoring of Free T4 levels helps ensure that thyroid hormone replacement therapy is within the therapeutic range, optimizing treatment outcomes and maintaining normal thyroid function.
4. Evaluation of Thyroid Dysfunction in Pregnancy: Free T4 is particularly useful in monitoring thyroid function during pregnancy. Thyroid hormone imbalances during pregnancy can impact the health of both the mother and the developing fetus. Measuring Free T4 levels aids in the diagnosis and management of thyroid disorders in pregnant women, helping to ensure appropriate thyroid hormone levels for optimal maternal and fetal well-being.
5. Assessment of Thyroid Function in Critical Illness: Free T4 is used in the evaluation of thyroid function in critically ill patients. Critical illness can affect thyroid hormone metabolism, resulting in altered Free T4 levels. Monitoring Free T4 levels helps healthcare providers assess thyroid function and guide appropriate interventions in critically ill individuals.
It is important to note that the interpretation of Free T4 levels requires consideration of other thyroid function tests, such as Free T3 and thyroid-stimulating hormone (TSH), as well as clinical presentation and patient history. Combining these results provides a more comprehensive assessment of thyroid function.
In summary, Free T4 has specific clinical significance in assessing thyroid function, diagnosing hypothyroidism, monitoring thyroid replacement therapy, evaluating thyroid dysfunction in pregnancy, and assessing thyroid function in critical illness. By measuring Free T4 levels in a patient's blood, this test provides valuable information for clinical decision-making, diagnosing and managing thyroid disorders, and optimizing patient care outcomes related to thyroid health.