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Unit Code 200154:
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 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 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 hypo-

thalamus 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 in patients with

acute thyroiditits.

 

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 hypo-

thyroidism.

 

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

chronic thyroiditis, and occasionally, subactue thyroiditis.

 

Increased total T4 is seen in pregnant patients and patients who are

on estrogen medication. These patients have increased total T4

levels due to 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 hyper-

thyroidism.

 

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 they physiologic amount of

T4.

 

Some patients who have been exposed to animal antigens, either in

the environment or as part of treatment or imaging procedures, 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. W.B. Saunders Company 1998

Key