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Relevant Laboratory And Instrumental Investigations In Determining Thyroid Hormones

Updated on February 9, 2014

T3 Resin Uptake

This test is a measure of the available binding sites in the TBG after combination with the patient’s own T3 and T4. Normal range is 25 to 35%. T3 resin uptake values are high in hyperthyroidism and low in hypothyroidism.
This test is a measure of the available binding sites in the TBG after combination with the patient’s own T3 and T4. Normal range is 25 to 35%. T3 resin uptake values are high in hyperthyroidism and low in hypothyroidism. | Source

Clinical Investigations

Such investigations are as follows: Radioactive iodine uptake test, resin uptake, free thyroxine index, estimation of T4, T3 and TSH, TRH stimulation test, Thyroid scintiscanning and thyroid antibodies determination.

Radioactive Iodine Uptake Test

The amount of radioactivity concentrated by the thyroid following oral administration of 131I, after 2 and 24 hours is measured. In Indians for instance, the normal ranges are 10+5% at 2 hours and 42+7% at 24 hours. The uptake values are high in hyperthyroidism and are low in hypothyroidism. Several other disorders and drugs interfere with this test and therefore reliance should not be made on this single test.

Normally, iodine uptake is suppressed by exogenous administration of T3 or T4. In Grave’s disease (primary hyperthyroidism) T3 and T4 fail to suppress iodine uptake. In hypothyroidism in order to identify the site of lesion as the thyroid, pituitary or hypothalamus, the effect of TSH or TRH respectively on iodine uptake by the thyroid is studied. In primary hypothyroidism, the values do not change, whereas in pituitary and hypothalamic disease, the iodine uptake is increased by TSH or TRH respectively.

T3 resin uptake (T3 RU)

This test is a measure of the available binding sites in the TBG after combination with the patient’s own T3 and T4. Normal range is 25 to 35%. T3 resin uptake values are high in hyperthyroidism and low in hypothyroidism. This test also loses its specificity since many drugs interfere with TBG. Estrogens increase and androgens decrease the binding sites in TBG. During pregnancy, T3 resin uptake is increased.

Free thyroxine Index (FTI)

FTI is the product of T4 or PBI values and T3 resin uptake expressed as a percentage ration. The values of PBI or T4 and T3RU may be altered in conditions which interfere with the synthesis of TBG, drug therapy and in pregnancy. However, under such circumstances, the alteration in PBI levels and T3RU values follow an inverse relationship, thereby keeping the product of PBI and T3 RU unaltered. For example, in patients who are on contraceptive pills (estrogens), the T4 values are increased and T3 RU is decreased, thereby keeping their products (FTI) normal. FTI is high in hyperthyroidism and low in hypothyroidism. The normal range of FTI levels have to be determined for each laboratory and this shows inter-laboratory variation.

Estimation of T4, T3 and TSH

These hormones are estimated by radioimmunoassay. Normal level of TSH in serum is 5 uU/ml. The changes in T3, T4 and TSH in different thyroid disorders are diagnostic.

 
T3
T4
TSH
Grave’s disease
High
High
Low
T3-toxicosis
High
Normal
Low
Primary hypothyroidism
Low
Low
Very High
Secondary and tertiary hypothyroidism
Low
Low
Low

Thyroid Scintiscanning

Originally I131 was used for visualizing the thyroid, the scan being done after 24 hours of administration. 99m Technitium pertechnetrate which has a half-life of 6 hours can also be used for this purpose
Originally I131 was used for visualizing the thyroid, the scan being done after 24 hours of administration. 99m Technitium pertechnetrate which has a half-life of 6 hours can also be used for this purpose | Source

More Clinical Investigations

TRH Stimulation Test: 100-200ug of TRH is injected intravenously after estimation of basal TSH level in the serum. At 20, 40 and 60 minutes. Post-injection, TSH levels are estimated. Normally, there is a two-fold rise. In Grave’s disease, the TSH response to TRH is minimal. In hypothalamic hypothyroidism, the basal level of TSH is low, but TRH causes the release of TSH. In pituitary disease, the basal as well as post-injection level of TSH remains the same at a low level.

Thyroid Scintiscanning: Scanning offers a visual display of the size and shape of the thyroid gland. It is employed in different situations.

  1. It provides an objective assessment of the morphology of the gland.
  2. It locates ectopic foci of thyroid tissue.
  3. The functional activity of thyroid nodules can be evaluated (hot or cold nodules).
  4. In the investigation of masses in the neck of mediastinum, scintiscanning is useful to detect their nature (e.g. retrosternal thyroid).
  5. Metastasis from thyroid carcinoma can be visualized.

Originally I131 was used for visualizing the thyroid, the scan being done after 24 hours of administration. 99m Technitium pertechnetrate which has a half-life of 6 hours can also be used for this purpose. In this case, the scan can be made soon after the injection of the radioisotope.

Finally, the different types of antibodies are demonstrable in patients with thyroid diseases. These include antibodies against thyroglobulin, intracellular microsomal antigen and nuclear components. Their presence is diagnostic in many disorders, especially chronic lymphocytic thyroiditis (Hashinomoto’s disease).

© 2014 Funom Theophilus Makama

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