Asma Shaukat, Samina Ghayur, Ejaz Hassan Khan, Farooq Ahmad Khan.
TSH as a screening marker for thyroid disease.
Pak J Pathol Jan ;7(2):10-4.

With the development of highly sensitive third generation TSH assay it is now possible to use TSH as a sensitive and specific test for the diagnosis of thyroid function abnormalities and as an aid in monitoring the adequacy of thyroid hormone replacement therapy. However, it is essential to establish analytical/clinical decision limits for TSH assay before TSH based screening strategy is implemented in any clinical setting. Hence a study was planned to determine the decision limits for TSH in order to have optimum sensitivity and specificity. One thousand serum samples were selected out of the departmental routine clinical work. Patients on therapeutic intervention for thyroid diseases were excluded from the study. Serum Free T4 and Total T3 were also measured in all the samples. These hormones were assayed by chemiluminescence immunoassay using Immulite autoanalyser. In 507 patients with TSH values falling between 0.73.5 mIU/L, serum FT4 and T3 values were within prescribed reference range for healthy population. The sensitivity of TSH for the diagnosis of hyperthyroidism at a concentration of <0.1 mIU/L was 99.25% and the specificity was 94.52%. Positive predictive value was 99.28% and negative predictive value was 99.86%. The sensitivity of TSH for the diagnosis of hypothyroidism at a concentration of >8 mIU/L was 100% and specificity was 94.52%. Positive predictive value was 89.28% and negative predictive value was 100%. It is concluded that satisfactory decision limits (cost-effective in terms of time and money) are possible for using third generation Immulite TSH assay. Subjects having TSH values between 0.7-3.5 m1U/L do not require FT4 and/or T3 estimation to rule out thyroid disease unless there is a history of recent threrapeutic intervention of thyroid disease. Similarly TSH values <0.1 mIU/L and >8mlU/L are the analytical decision limits where hyperthyroidism and hypothyroidism respectivley are mostly confirmed except in situation where pituitary is primarily responsible for thyroid disease. Subjects having TSH values between 0.1-0.7 mIU/L and between 3.5-8.0 mIU/L should be further evaluated by measuring FT4 and / or T3. This strategy will help in reducing the un-necessary measurement of FT4/T3 in about 60% cases.

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