free thyroxine


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Related to free thyroxine: free thyroxine index, Free Triiodothyronine

thyroxine

 (T4) [thi-rok´sin]
a thyroid hormone that contains iodine and is a derivative of the amino acid tyrosine, occurring naturally as l-thyroxine; its chemical name is tetraiodothyronine. It is formed and stored in the thyroid follicles as thyroglobulin and released from the gland by the action of a proteolytic enzyme. It is deiodinated in peripheral tissues to form triiodothyronine (T3), which has a greater biological activity.



Thyroxine acts as a catalyst in the body and influences a great variety of effects, including metabolic rate (oxygen consumption); growth and development; metabolism of carbohydrates, fats, proteins, electrolytes, and water; vitamin requirements; reproduction; and resistance to infection. Pharmaceutical preparations of thyroxine, extracted from animals or made synthetically, are called levothyroxine.
free thyroxine the fraction of thyroxine in the serum that is not bound to a binding protein.

free thyroxine

the amount of the unbound, active thyroid hormone thyroxine circulating in the blood, measured by specific laboratory procedures. See also free thyroxine index.

thyroxine, thyroxin

a hormone of the thyroid gland that contains iodine and is a derivative of the amino acid tyrosine. The chemical name for thyroxine is tetraiodothyronine (symbol, T4); it is formed and stored in the thyroid follicles as thyroglobulin, the storage form. Thyroxine is released from the gland by the action of a proteolytic enzyme. T4 is deiodinated in peripheral tissues to form tri-iodothyronine (T3), which has a greater biological activity.
Thyroxine acts as a catalyst in the body and influences a great variety of effects, including metabolic rate (oxygen consumption); growth and development; metabolism of carbohydrates, fats, proteins, electrolytes and water; vitamin requirements; reproduction; and resistance to infection.
Thyroxine can be extracted from animals or made synthetically; it is used in the treatment of hypothyroidism and some types of goiter.

free thyroxine
the metabolically active fraction of thyroxine; abbreviated FT4, Tf. T4 = Tf + TBG. The amount is very small and difficult to estimate so that the amount present in serum is not used as a more accurate indicator of thyroxine status than T4.
thyroxine-binding globulin (TBG)
most (99.95%) of the thyroxine in plasma is bound to globulin and a small amount bound to prealbumin.
thyroxine-binding prealbumin
bound to a small fraction of circulating T4. This is the only known function of prealbumins.
L-thyroxine
References in periodicals archive ?
Free thyroxine and free triiodothyronine," Clinical Chemistry, vol.
A patient with pseudo-Addison's disease and falsely elevated thyroxine due to interference in serum cortisol and free thyroxine immunoassays by two different mechanisms.
Measurement of serum total thyroxine, triiodothyronine, free thyroxine, and thyrotropin concentrations for diagnosis of hypothyroidism in dogs.
Patients with high free thyroxine level (FT4) and a suppressed thyroid stimulating hormone (TSH) level and those on antithyroid medication were defined as being hyperthyroid, those with low FT4 or on thyroxine as hypothyroid and those with normal FT4 and a normal or low TSH as euthyroid.
When the participants took the medication at night, their thyrotropin levels decreased and their free thyroxine and total triiodothyronine levels increased.
He presented preliminary data on the CATS study population comprising nearly 22,000 women with singleton pregnancies whose blood was tested for free thyroxine (T4) and thyroid-stimulating hormone before 16 weeks' gestation.
Williams and his colleagues reported that higher levels of free thyroxine (T4) and triiodothyronine (T3) were associated with significantly lower BMD at the hip, and higher free T4 was associated with increasing bone loss at the hip.
1 IGF-1: insulin-like growth factor-1; FT3: free triiodothyronine; FT4: free thyroxine.
The study aimed to determine and compare the usefulness of cord blood screening for free thyroxine (FT4) and thyroid stimulating hormone (TSH).
What are the advantages and disadvantages of replacing a calculated a free thyroxine index (FTI) with a directly determined free T4.
Reference intervals for free thyroxine and free triiodothyronine measured by mass spectrometry are now also included.