Triiodothyronine uptake test

M7. Mardell, R. J., A source of error in the automated serum triiodothyronine-uptake test. Clin. Chem. 16, 951-953 (1970). 

Drug/Lab test interactions Certain endocrine tests may be affected by levonorgestrel implants Sex hormone binding globulin concentrations are decreased thyroxine concentrations may be slightly decreased and triiodothyronine uptake increased. 

Sterling, K., and Tabachnick, M., Resin uptake of triiodothyronine as test of thyroid function. J. Clin. Endocrinol. Metab. 21, 456-464 (1961). 

A high radioactive iodine neck uptake test (q.v.) is usually indicative of hyperthyroidism, although a high uptake can occur if the patient is iodine deficient or is taking anti-thyroid drugs. The neck uptake test can therefore be repeated after triiodothyronine (T3) administration. This normally suppresses TSH secretion, resulting in a low uptake. However, in hyperthyroidism but not in the other conditions listed above, no such suppression occurs. 

Artefactual increases of as much as 50% in total thyroxine, estimated by a competitive protein-binding assay, and of as much as 30% in triiodothyronine resin uptake are probably due to rapid and continuing lipolytic hydrolysis of triglycerides after blood has been drawn (126). Thyroid function tests should therefore always be performed on blood samples taken before (or a sufficient time after) heparin treatment (127). An increase in serum-free thyroxine concentrations has also been reported after low molecular weight heparin, by up to 171% in specimens taken 2-6 hours after injection. When specimens were obtained 10 hours after injection, the effects were smaller, but with concentrations still up to 40% above normal the results can still cause errors of interpretation (128). 

The mechanism by which TRF controls the release of TSH from the pituitary is discussed in Section 2.4. It has been known for several years that the administration of thyroxine (T4) or triiodothyronine (T3) would inhibit the uptake of radioiodine by the thyroids of normal individuals. The administered hormone presumably acted by decreasing the secretion of TSH by the pituitary. This action is the basis of the T3 suppression test described by Werner (Wl). T3 is given to the patient for 7 days. In the normal individual, I uptake by the thyroid will be... 

Hamolsky, M. W., A. Golodetz, and A. S. Freedberg. 1959. The plasma protein-thyroid hormone complex in man. IB. Further studies on the use of the in vitro red blood cell uptake of Ij3j triiodothyronine as a diagnostic test of thyroid function. Journal of Clinical Endrocrinology and Metabolism 19 101-116. 

W17. Wemer, S. C., and Spooner, M., A new and simple test of hypeTth Toidism employing 1-triiodothyronine and the twenty-four-hour 1-131 uptake method. Bull. N.Y. Acad. Med. [2] 31, 137-145 (1955). 

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