Thyroid function tests 1

TESTS OF THYROID FUNCTION

      T4, T3 estimations by radioimmunoassay are direct assessment of total T4, T3. These values correlate well with the functional status of the patient in most cases. These estimation can now also be done by ELISA technique.

      In case the thyroxine binding globulin (TBG) is high i.e. in those on contraceptive pills, there may be high values with normal function. Normal value of

-     T4 is 4-12 µg/100 ml

-     T3 is 100-200 µg/100 ml

-     Free T4+T3 can also be of value

4.1 Serum TSH Estimation

      This is a very sensitive test of thyroid function, since it responds to circulating levels of T4+T3. In cases of hypothyroidism, the TSH value is high.

Normal values range from 0-5 micro-units/ml

4.2 Iodine Studies

      Since iodine is the principal element required in the biosynthesis of T4,T3 any study of iodine content of serum and iodine handling by the thyroid would provide an indirect evidence regarding thyroid function.

4.3 Protein Bound Iodine

      In the past in the absence of reliable estimations of T4+T3 Protein Bound Iodine (PBI) provided indirect evidence regarding thyroid function.

*     Nearly 99% of thyroxine (T4) and othe iodothyronines in blood are bound to TBG and thus measuratleas PBI.

*     The normal range of PBI is 3.5-7.5 µg/100 ml

*       Unfortunately the level of TBG increases with the increase of oestrogens (Pill, Pregnancy and Liver disease). Oral intake of iodides (Expectorants, Iodo-hydroxy quinilones and X-ray Contrast Media) can also raise PBI, thus rendering this test unreliable

4.4 I131 Uptake

      The percent of I131 taken up by the gland at 2 and 24 hours can be compared with the uptake in normal subjects in any given country.

*     High uptake can be due to iodine deficient diet or hyperthyroidism

*       Unfortunately, this test result can be altered by intake of iodides (Expectorants and othe
rs)

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