CPT code 84436 is the standardized medical billing code used for the Thyroxine (T4), Total laboratory test. CPT (Current Procedural Terminology) provides a uniform system for describing medical services for administrative and financial purposes. This specific code identifies a blood test that measures the entire amount of thyroxine hormone circulating in a patient’s bloodstream. The Thyroxine (T4), Total test helps physicians assess the function of the thyroid gland, a small, butterfly-shaped organ located in the neck.
The Thyroxine Hormone and the Measurement
Thyroxine, or T4, is the main hormone produced and secreted by the thyroid gland, a process controlled by the pituitary gland’s Thyroid-Stimulating Hormone (TSH). T4 plays a significant part in regulating the body’s metabolism, affecting how the body uses energy, and influencing functions like heart rate, digestion, and body temperature. T4 is considered a prohormone because a substantial portion of it is converted in the peripheral tissues into triiodothyronine (T3), which is the most biologically active thyroid hormone.
The T4 hormone exists in two forms within the blood: bound and free. The vast majority of T4 is “bound” to specific proteins, primarily thyroxine-binding globulin, which prevents it from entering body tissues and acts as a circulating reserve. A very small fraction, around 0.05%, remains “free” and can easily enter the cells to exert its biological effects.
The CPT 84436, or Total T4, measures both the bound and the free fractions of the hormone to determine the overall concentration in the blood. While this provides a general picture of thyroid hormone production, the measurement of Total T4 can be influenced by changes in the level of the binding proteins. Conditions like pregnancy, liver disease, or taking certain medications can increase these binding proteins, which would artificially elevate the Total T4 level even if the active free hormone level remains normal. For this reason, the Free T4 test is often preferred as it measures only the unbound, active hormone, offering a more direct assessment of thyroid status.
Clinical Indications for Testing
A physician orders a Thyroxine (T4), Total test when there is suspicion of thyroid dysfunction based on a patient’s symptoms or as a follow-up to an abnormal TSH test. The test helps diagnose conditions where the thyroid is either overactive (hyperthyroidism) or underactive (hypothyroidism). These disorders manifest through non-specific symptoms related to a slowed or accelerated metabolism.
Symptoms suggesting hypothyroidism include persistent fatigue, unexplained weight gain, dry skin, and a slow heart rate. Conversely, signs of hyperthyroidism involve unexplained weight loss, rapid or irregular heartbeat, increased anxiety, difficulty sleeping, and heat intolerance.
The Total T4 test is also used to monitor the effectiveness of treatment in patients already diagnosed with a thyroid disorder. In some cases, it may be used to screen newborns for congenital hypothyroidism or to evaluate problems related to the pituitary or hypothalamus glands, which control thyroid function.
Understanding High and Low Results
An abnormal Total T4 result suggests the thyroid gland is not producing the correct amount of hormone, but interpretation must be done in conjunction with other tests like TSH and Free T4. A low Total T4 level suggests hypothyroidism, meaning the thyroid gland is underactive and producing insufficient hormone. This low level is commonly linked to conditions such as Hashimoto’s disease or can occur after medical treatments like thyroid surgery.
Conversely, an elevated Total T4 level indicates hyperthyroidism, where the thyroid is overactive and producing too much thyroxine. This excessive hormone production may be caused by conditions like Graves’ disease or by toxic thyroid nodules. High Total T4 can also be caused by non-thyroid issues, such as high levels of binding proteins due to factors like pregnancy or certain medications.
When Total T4 results are abnormal, further testing is required to confirm the diagnosis and identify the root cause. The healthcare provider uses the pattern of TSH and Free T4 results to differentiate between primary thyroid disease and pituitary gland issues. For instance, a high TSH and low Free T4 confirm primary hypothyroidism, while a low TSH and high Free T4 points to primary hyperthyroidism.