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Unit Code 8724:
T4 (Thyroxine), Total Only, Serum

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Useful For

Monitoring treatment with synthetic hormones (synthetic T3 will

cause a low total T4)

 

Monitoring treatment of hyperthyroidism with thiouracil and other antithyroid

drugs

 

Total T4 levels offer a good index of thyroid function when the TBG

is normal and nonthyroidol illness is not present.

Clinical Information

Thyroxine 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 (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 triiodothyronine (T3). High amounts of T4

and T3 (mostly from peripheral conversion of T4) cause hyper-

thyroidism.

 

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 Algorithm" in Special Instructions.

Reference Values

>=1 year: 5.0-12.5 ug/dL

Interpretation

Values >12.5 ug/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 ug/dL are generally signified as normal in

the literature and values <12 ug/dL for a newborn may signify

hypothyroidism.

 

Values <5.0 ug/dL are seen in hypothyroidism, myxedema,

cretinism, chronic thyroiditis, and occasionally, subacute thyroiditis.

 

Increased total T4 is seen in pregnancy and patients who are on

estrogen medication. These patients have increased total T4 levels

(increased 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 TSH, TBG, or free T4

testing is needed.

Cautions

TSH may be better than T4 as the initial test of thyroid status. TSH

is elevated in primary hypothyroidism. TSH is low in primary hyper-

thyroidism.

 

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.

Special Instructions and Forms

• Thyroid Function Algorithm 

Clinical Reference

Wilson JD, Foster DW, Kronenburg MD, et al:  Williams textbook

of Endocrinology. 9th edition, WB Saunders Company, 1998

Key