The thyroid gland is a butterfly-shaped gland that lies in front of your windpipe ( trachea just below your voice box (larynx). The thyroid gland uses iodine from food to make two thyroid hormones : thyroxine ( T4 ) and triiodothyronine ( T3 ).
The United States Preventive Services Task Force recommends that all newborns be tested for congenital hypothyroidism. 1 Thyroid hormone blood tests include: Total thyroxine (T4). Most of the thyroxine (T4) in the blood is attached to a protein called thyroxine-binding globulin.
Adult g/dL Pregnancy 1st Trimester g/dL 2nd Trimester g/dL 3rd Trimester g/dL All Trimesters Together g/dL Clinical Significance. Thyroxine (T4) is the major secretory hormone of the thyroid. Only 0.03 of T4 is unbound and free for exchange with tissues.
A. Direct FT4 and FT3 Methods. Direct free hormone methods employ equilibrium dialysis (10,95-97 ultrafiltration (9,11,86,98-101) or gel filtration (102) to separate the exceeding small amount of free hormone from the dominant protein-bound moiety.
Screen newborns to find out if the thyroid gland function is normal. A condition called congenital hypothyroidism can prevent normal growth and development and cause other severe problems, such as intellectual disability, if it is not treated soon after birth.
Although the diagnostic accuracy of free hormone methods cannot be predicted either from a methods classification or by an in-vitro test of technical validity such as dilution (27,83,89-91 the superior log/linear TSH/FT4 relationship seen with direct FT4 methods (ultrafiltration followed by LC-MS/MS) highlights the inferiority of current FT4 immunoassays (11,92,93).
The FTI can help tell if abnormal amounts of T4 are present because of abnormal amounts of thyroxine-binding globulin. Triiodothyronine (T3). Most of the T3 in the blood is attached to thyroxine-binding globulin.
The thyroid gland stores these thyroid hormones and releases them as they are needed. Thyroid hormones are needed for normal development of the brain, especially during the first 3 years of life.
Therefore, total T4 should not be used as the only marker for thyroid function evaluation. Thyroid-stimulating hormone (TSH) may be better than thyroxine (T4) as the initial test of thyroid status.
Less than 1 of the T4 is unattached. A total T4 blood test measures both bound and free thyroxine. Free thyroxine affects tissue function in the body, but bound thyroxine does not.
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 nephrosis (decreased TBG levels). A thyrotropin-releasing hormone stimulation test may be required for certain cases of hyperthyroidism.
In addition, measurements can be made of the thyroid hormone binding proteins, Thyroxine Binding Globulin (TBG Transthyretin (TTR Prealbumin (TBPA ) and Albumin (18). The recognition that autoimmunity represents a major cause of thyroid dysfunction has led to the development of tests for the detection of thyroid autoautoantibodies such as thyroid peroxidase antibodies (TPOA b.
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.
Over the past forty years, improvements in the sensitivity and specificity of thyroid testing methodologies have dramatically impacted clinical strategies for detecting and treating thyroid disorders. In the 1950s, only one thyroid test was available an indirect estimate of the serum total (free protein-bound) thyroxine (T4) concentration, using the protein bound iodine (PBI) technique (1,2).
TThe diagnostic accuracy of total hormone measurements would be proportional to that of free hormone tests if all patients had similar binding protein concentrations (18,27,64). For example, a recent study has reported that a screening cord blood TT4 7.6 ug/dL ( 98 nmol/L) may serve as a valid screening test for congenital hypothyroidism (65).
Recently, primary T4 and T3 calibrator standards have become available, and measurements based on LC-MS/MS have further improved the standardization of these tests (10,28). Most commonly, TT4 and TT3 concentrations are measured by competitive non-isotopic immunoassay methods performed on automated platforms that use enzymes, fluorescence or chemiluminescent molecules as signals (14,25 27).