The Thyroid Gland
The thyroid gland is part of the endocrine system and is located in the front of the neck. It produces thyroid hormones that are used for metabolism. The thyroid hormones consist of T3 or triiodothyronine and T4 or thyroxine. Total T3 and Total T4 represent the forms of thyroid hormones that are bound to a protein in the body such as albumin. Free T3 (FT3) and Free T4 (FT4) are the thyroid hormones that are not bound to a protein in the body.
Thyroid hormones are released from the thyroid in the presence of thyroid stimulating hormones (TSH) produced by the pituitary gland. The level of TSH is also measured in thyroid laboratory tests, and is a strong indicator for Hypothyroidism or Grave’s Disease.
Normal Thyroid Lab Test Reference Range
Here are the normal ranges for thyroid tests according to Rush University Medical Center:
TSH = 0.350 - 4.940 µIU/ml
Total T3 = 0.6 - 1.6 ng/ml
Total T4 = 4.9 - 11.7 mg/dl
Free T3 = 1.7 - 3.7 pg/ml
Free T4 = 0.7 - 1.5 ng/dl
Using Laboratory Test Results
The laboratory test for hypoactive or hyperactive thyroid is used to test if a person has Hypothyroidism or Grave’s disease. Even if laboratory results are out of range, a physician is more inclined to administer treatment based on the person’s signs and symptoms on medical examination.
Like all laboratory tests, the thyroid lab test can produce results that are false positive or false negative. A false negative will produce a result telling someone they do not have a medical condition when indeed they do. However, a false positive is the exact opposite and someone will be told they have a condition when they do not.
Therefore, it is possible for someone to be within the lower end of the normal range for Free T3 and still be showing symptoms of low thyroid hormone. Thus a physician cannot always rely on just the laboratory results for a patient and instead will consider treatment based on the “whole person” after conducting a thorough physical examination.
When reading the reference ranges it is also a good idea to keep in mind that a laboratory can report the thyroid results either in milligrams (mg), nanograms (ng), and picograms (pg) depending on what is being tested. The presence of autoimmune antibodies to the thyroid gland can greatly alter the normal ranges.
Web Source: American Association for Clinical Chemistry. “T3: The Test.” 2010. Available at: https://www.labtestsonline.org/understanding/analytes/t3/test.html
Web Source: American Association for Clinical Chemistry. “Thyroid Antibodies.” 2010. Available at: https://www.labtestsonline.org/understanding/analytes/thyroid_antibodies/test.html
Web Source: Melissa Breedlove, Davidson College. “Graves Disease.” 2000. Available at: https://www.bio.davidson.edu/Courses/Immunology/Students/Spring2003/Breedlove/GravesDisease.html
Web Source: Rush University Medical Center: “Normal Ranges for Common Laboratory Tests: Thyroid Tests.” 2010. Available at: https://www.rush.edu/webapps/rml/RMLRangesThyroid.jsp