T4 (Thyroxine), Total Only, Serum
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Thyroxine (T4) is a metabolically active thyroid hormone containing 4 atoms of iodine (tetraiodothyronine: T4). About 0.05% of circulating T4 is in the free or unbound portion. The remainder is bound to thyroxine-binding globulin (TBG) prealbumin and albumin.
The hypothalamus secretes thyrotropin-releasing hormone (TRH), which stimulates the pituitary to release thyroid-stimulating hormone (TSH). TSH stimulates the thyroid to secrete T4. T4 is partially converted peripherally to triidothyronine (T3). High amounts of T4 and T3 (mostly from peripheral conversion of T4) cause hyperthyroidism.
T4 and T3 cause positive feedback to the pituitary and hypothalamus with resultant suppression or stimulation of the thyroid gland as follows: decrease of TSH if T3 or T4 is high (hyperthyroidism), and increase of TSH if T3 or T4 is low (hypothyroidism).
See Thyroid Function Ordering Algorithm in Special Instructions.
Monitoring treatment with synthetic hormones (synthetic triiodothyronine will cause a low total thyroxine: T4)
Monitoring treatment of hyperthyroidism with thiouracil and other antithyroid drugs
Total T4 levels offer a good index of thyroid function when the thyroid-binding globulin is normal and nonthyroidal illness is not present.
Values >12.5 mcg/dL are seen in hyperthyroidism and patients with acute thyroiditis.
The normal concentration for newborns is much higher than for adults. Values up to 23 mcg/dL are generally signified as normal in the literature and values <12 mcg/dL for a newborn may signify hypothyroidism.
Values <5.0 mcg/dL are seen in hypothyroidism, myxedema, cretinism, chronic thyroiditis, and occasionally, subacute thyroiditis.
Increased total thyroxine (T4) is seen in pregnancy and patients who are on estrogen medication. These patients have increased total T4 levels (increased thyroxine-binding globulin [TBG] levels).
Decreased total T4 is seen in patients on treatment with anabolic steroids, or patients with chronic liver disease, or nephrosis (decreased TBG levels).
A TRH stimulation test may be required for certain cases of hyperthyroidism.
Clinical findings are necessary to determine if thyroid-stimulating hormone, TBG, or free T4 testing is needed.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
In pregnancy, total T4 might be falsely-low with this assay and should not be used as the only marker for thyroid disease evaluation.
Thyroid-stimulating hormone (TSH) may be better than thyroxine (T4) as the initial test of thyroid status. TSH is elevated in primary hypothyroidism. TSH is low in primary hyperthyroidism.
Free T4 may more accurately measure the physiologic amount of T4.
Some patients who have been exposed to animal antigens, either in the environment or as part of treatment or imaging procedure, may have circulating antianimal antibodies present. These antibodies may interfere with the assay reagents to produce unreliable results.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
> or =12 months: 5.0-12.5 mcg/dL
Reference values have not been established for patients that are <12 months of age.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Wilson JD, Foster DW, Kronenburg MD, et al: Williams Textbook of Endocrinology. Ninth edition. WB Saunders Company, 1998