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Thyroid function tests

Three tests are commonly referred to as thyroid function tests:

  • Free T4
  • T3
  • TSH

Free T4, serum

Free T4 is the metabolically active fraction of the total T4 level (the other fraction is bound to plasma protein) and is the precursor of T3.

The T4 reflects the amount of thyroxine in the blood. If the patient does not take any type of thyroid medication, this test is usually a good measure of thyroid function.

Methodology

A blood sample is taken and serum is separated within 48 hours. T4 is done by RIA (radioimmunoassay) is the most used thyroid test of all. Other methods used are fluorescent immunoassay and chemiluminescent immunoassay. A resin T3 uptake (RT3u) should be done to correct for certain medications such as birth control pills, other hormones, seizure medication, cardiac drugs, or even aspirin that may alter the routine T4 test.

Reference levels

  • Newborns: 2.6 - 6.3 ng/dL (SI: 33.5 - 81.3 pmol/L)
  • Adults: 0.8 - 2.7 ng/dL (10.3-35 pmol/L)

T3, serum

Thyroxine (T4) represents 80% of the thyroid hormone produced by the normal gland and generally represents the overall function of the gland. The other 20% is triiodothyronine measured as T3. Sometimes the diseased thyroid gland will start producing very high levels of T3 but still produce normal levels of T4. Therefore measurement of both hormones provides an even more accurate evaluation of thyroid function.

T4 is the precursor of T3 and is less bound to plasma protein (thyroid binding globulin) and hence is more biologically active.

Methodology

A blood sample is taken and serum is separated within 48 hours. T3 is done by RIA (radioimmunoassay). Other methods include fluorescence polarization immunoassay (FPIA) and fluorometric immunoassay.

Reference levels

  • Adults (20 - 50 years old): 70 - 204 ng/dL (SI: 0.62 - 2.79 nmol/L)


TSH, serum

Normally, low levels (less than 5 units) of TSH are sufficient to keep the normal thyroid gland functioning properly. When the thyroid gland becomes inefficient such as in early hypothyroidism, the TSH becomes elevated even though the T4 and T3 may still be within the "normal" range. This rise in TSH represents the pituitary gland's response to a drop in circulating thyroid hormone; it is usually the first indication of thyroid gland failure. Since TSH is normally low when the thyroid gland is functioning properly, the failure of TSH to rise when circulating thyroid hormones are low is an indication of impaired pituitary function. Sensitive TSH tests will show very low levels of TSH when the thyroid is overactive (as a normal response of the pituitary to try to decrease thyroid stimulation). Interpretations of the TSH level depends upon the level of thyroid hormone; therefore, the TSH is usually used in combination with other thyroid tests such as the T4 RIA and T3 RIA.

Methodology

A blood sample is taken and serum is separated within 48 hours. A diurnal rhythm exists with peak TSH levels occuring at about 11 PM. Pituitary production of TSH is usually measured by a method referred to as IRMA (immunoradiometric assay).

Reference levels

  • Adults (21 - 54 years): 0.4 - 4.2 mIU/L

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