Free T3, Free T4, RT3 and TSH Panel

The Free T3, Free T4, RT3 and TSH Panel panel contains 4 tests with 5 biomarkers.

Brief Description: The Free T3, Free T4, RT3, and TSH Panel is a comprehensive thyroid function test that measures the levels of free triiodothyronine (T3), free thyroxine (T4), reverse triiodothyronine (RT3), and thyroid-stimulating hormone (TSH) in the blood. This panel provides a detailed view of thyroid health, offering insights into thyroid hormone production, metabolism, and the pituitary gland's regulatory role.

Also Known As: Thyroid Stimulating Hormone Test, Thyrotropin Test, TSH test, Thyroid Test, TSH Screen Test

Collection Method: Blood Draw

Specimen Type: Serum

Test Preparation: Specific to TSH: Specimen collection after fluorescein dye angiography should be delayed for at least 3 days. For patients on hemodialysis, specimen collection should be delayed for 2 weeks.

According to the assay manufacturer Siemens: "Samples containing fluorescein can produce falsely depressed values when tested with the Advia Centaur TSH3 Ultra assay."

When and Why the Free T3, Free T4, RT3, and TSH Panel May Be Ordered

Healthcare providers may order this panel when symptoms suggest thyroid dysfunction, such as unexpected weight changes, mood swings, changes in energy levels, and alterations in heart rate. It's also used to monitor the effectiveness of thyroid disorder treatments and to assess the balance and conversion of thyroid hormones in the body.

What the Free T3, Free T4, RT3, and TSH Panel Checks For

  • Reverse T3 (RT3): RT3 is an inactive form of T3. It's produced in the body from T4 and can increase during periods of stress, illness, or with certain medications. High levels can indicate that the body is converting too much T4 into RT3 instead of active T3, potentially leading to symptoms of hypothyroidism despite normal T4 and TSH levels.
  • T3 Free (Free Triiodothyronine): Measures the free, or unbound, T3 hormone available to body tissues. It provides information about the body's metabolic state and can help diagnose hyperthyroidism or hypothyroidism, especially in situations where T4 levels are normal but symptoms persist.
  • T4 Free (Free Thyroxine): Assesses the free T4 hormone, which is not bound to proteins in the blood and is available to enter and affect various tissues. It is a key hormone produced by the thyroid gland, and its levels can indicate thyroid gland function.
  • Thyroid Stimulating Hormone (TSH): Produced by the pituitary gland, TSH regulates the production of T3 and T4 by the thyroid gland. It's often the first test done to evaluate thyroid function, with high levels suggesting hypothyroidism and low levels indicating hyperthyroidism.

Conditions and Diseases Detected by the Free T3, Free T4, RT3, and TSH Panel

This panel can help diagnose and manage several thyroid-related conditions:

  • Hypothyroidism: Characterized by high TSH and low free T4 levels, indicating an underactive thyroid.
  • Hyperthyroidism: Marked by low TSH and high free T4 or free T3 levels, suggesting an overactive thyroid.
  • Euthyroid Sick Syndrome: Normal or low TSH with low free T3 and potentially elevated RT3, seen in non-thyroidal illnesses.
  • Subclinical Hypothyroidism or Hyperthyroidism: Normal free T4 and free T3 levels with mildly elevated or decreased TSH levels, respectively.

Using the Free T3, Free T4, RT3, and TSH Panel Results in Clinical Practice

Healthcare professionals use the results from this panel to:

  • Diagnose Thyroid Disorders: Identifying specific patterns of thyroid hormone levels and TSH to diagnose various forms of thyroid dysfunction.
  • Monitor Treatment: Adjusting thyroid hormone replacement or suppressive therapy based on hormone levels and TSH.
  • Evaluate Hormone Conversion: Assessing the conversion efficiency of T4 to T3 and the potential diversion to RT3, which can influence treatment decisions.

The Free T3, Free T4, RT3, and TSH Panel offers a comprehensive assessment of thyroid function, providing essential insights for diagnosing, treating, and monitoring thyroid disorders. By measuring both the active and inactive forms of thyroid hormones along with the regulatory hormone TSH, this panel enables healthcare providers to understand the complex interplay of thyroid health and its impact on the body. This thorough approach ensures that patients receive accurate diagnoses and effective treatments tailored to their specific thyroid health needs.

We advise having your results reviewed by a licensed medical healthcare professional for proper interpretation of your results.

The following is a list of what is included in the item above. Click the test(s) below to view what biomarkers are measured along with an explanation of what the biomarker is measuring.

Also known as: Reverse T3, Reverse Triiodothyronine, RT3, T3 Reverse RT3 LCMSMS, Triiodothyronine Reverse

T3 Reverse, LC/MS/MS

Reverse T3 produced in the thyroid comes from the conversion of the storage hormone T4. Your body, especially the liver, can constantly be converting T4 to RT3 as a way to get rid of any unneeded T4. In any given day approx. 40% of T4 goes to T3 and 20% of T4 goes to Reverse T3. However in any situation where your body needs to conserve energy and focus on something else, it will change the above percentages, changing the conversion of RT3 to 50% or more, and the T3 goes down, down. Examples are emotional, physical, or biological stress, such as being chronically or acutely sick (the flu, pneumonia, etc), after surgery, after a car accident or any acute injury, chronic stress causing high cortisol, being exposed to an extremely cold environment, diabetes, aging, or even being on drugs like beta blockers and amiodarone.

Also known as: Free T3, FT3, T3 Free

T3, Free

This test measures the amount of triiodothyronine, or T3, in the blood.

Also known as: Free T4, FT4, T4 Free

T4, Free

The free T4 test is not affected by protein levels. Since free T4 is the active form of thyroxine, the free T4 test is may be a more accurate reflection of thyroid hormone function.

Also known as: Thyroid Stimulating Hormone, Thyroid Stimulating Hormone (TSH), Thyrotropin


A TSH test is a lab test that measures the amount of thyroid stimulating hormone (TSH) in your blood. TSH is produced by the pituitary gland. It tells the thyroid gland to make and release thyroid hormones into the blood.


*Process times are an estimate and are not guaranteed. The lab may need additional time due to weather, holidays, confirmation/repeat testing, or equipment maintenance.

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