Thyroxine, Total (Total T4)
CPT Code(s):
84436
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:
Plasma specimen; gross hemolysis; lipemia; improper labeling
Stability Criteria
Room Temperature:
4 days
Refrigerated:
14 days
Frozen:
183 days
Clinical Significance
T4, also known as thyroxine test, is a diagnostic tool used to measure the levels of thyroxine (T4) in a patient's blood. T4 is one of the two primary thyroid hormones produced by the thyroid gland and plays a crucial role in regulating metabolism, growth, and development. T4 has specific clinical significance and diagnostic use in the following ways:
1. Evaluation of Thyroid Function: T4 is commonly performed to assess thyroid function and diagnose thyroid disorders. Abnormal T4 levels can indicate an overactive thyroid (hyperthyroidism) or an underactive thyroid (hypothyroidism). This test is often used in combination with other thyroid function tests, such as T3 and thyroid-stimulating hormone (TSH) test, to provide a comprehensive evaluation of thyroid health.
2. Diagnosis and Monitoring of Hypothyroidism: Hypothyroidism is a condition characterized by insufficient production of thyroid hormones. T4 is a primary tool for diagnosing hypothyroidism and monitoring the effectiveness of treatment. Low T4 levels, along with elevated TSH levels, indicate an underactive thyroid and guide healthcare providers in determining appropriate treatment strategies.
3. Assessment of Hyperthyroidism: Hyperthyroidism is a condition characterized by excessive production of thyroid hormones. T4 helps detect elevated T4 levels, along with suppressed or low TSH levels, which are indicative of an overactive thyroid. Monitoring T4 levels in patients with hyperthyroidism assists in evaluating the effectiveness of treatment and adjusting medication dosage.
4. Differential Diagnosis: T4 levels are crucial in differentiating between different types of thyroid disorders. For example, elevated T4 levels with normal or low TSH levels can suggest a condition known as T4 toxicosis or thyrotoxicosis without hyperthyroidism, which requires further investigation and management.
5. Monitoring Thyroid Replacement Therapy: Patients receiving thyroid hormone replacement therapy, such as those with hypothyroidism, undergo regular T4 testing to ensure that hormone levels are within the therapeutic range. Monitoring T4 levels helps healthcare providers adjust medication dosage and maintain optimal thyroid hormone levels.
6. Newborn Screening: T4 testing is included in newborn screening programs to detect congenital hypothyroidism early. Low T4 levels in newborns can indicate an underactive thyroid, allowing for prompt intervention and management.
In summary, T4 is a valuable tool for evaluating thyroid function, diagnosing thyroid disorders, monitoring thyroid hormone replacement therapy, and guiding treatment decisions. It provides essential information for healthcare providers to assess the functioning of the thyroid gland and optimize patient care. Interpretation of T4 results, in conjunction with other thyroid function tests, aids in the accurate diagnosis and management of thyroid-related conditions.