T4 converting to T3 hormone occurs when one iodine atom is removed from T4 to result in T3 hormone. T3 hormone is active thyroid hormone that every cell of the body is dependent upon to function. T4 that travels througout the rest of the body must be converted to T3 hormone at the cellular level. A negative feedback loop is created that results in a stable release of TSH from the pituitary and T4 from the thyroid gland. The pituitary gland responds to levels of T4 in the blood by lowering TSH that is secreted, but not too much. T4 hormone travels from the thyroid gland back to the pituitary gland as well as throughout the whole body. T4 and T3 are secreted in an 11:1 ratio ( Snyder, 2012) from the thyroid gland in humans. It also secretes some T3 (triiodothyronine) hormone. TSH docks with receptors on the thyroid gland, triggering the gland to secrete T4 (thyroxine) hormone. It's released into the blood stream where it travels to the thyroid gland. TSH (thyroid stimulating hormone) is secreted from the pituitary gland. I'll start by reviewing in very simple terms the thyroid hormone cascade. The other labs that are pertinent to thyroid hormone function are CBC, ferritin, zinc, copper and vitamin D. There are others but these are values that assess direct thyroid hormone status. The six most important labs to obtain when assessing thyroid function, regardless of the age of the patient, are: Ultimately, a well-studied physician should have their own reference ranges that they use when evaluating labs. This has resulted in sometimes huge variations in reference ranges used by different labs. Unfortunately, there are no national or international standards for reference ranges that labs use. This is an important topic as thyroid hormone is crucial for brain and global development of infants and children. This question comes up a lot online: "What are the optimal reference ranges for thyroid hormone labs in children?" I hope this post serves to help parents and physicians understand optimal reference ranges for children versus reference ranges reported on actual lab results.
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