The Choice and Uses of Tests in Special Situations

In this section, an attempt will be made to rationalize the use of thyroid function tests in various clinical areas, particularly in those where interpretation can sometimes be diflScult. It is superfluous here to discuss in any detail the use of thyroid tests in certain clear-cut areas, such as in confirming the diagnosis in a newly presenting case of Graves disease or of idiopadiic myxedema. What is indicated is a standard regime of either an FTI or ETR (or similar test) or even both. If these measurements are in the overlap region between hypothyroid and normal subjects, a TSH assay is needed. If the routine tests are in the overlap between values for hyperthyroid patients and normal subjects, then a serum Ts concentration is indicated. 

The otfier screening tests to be performed are serum total T and FBI estimations. The combination of a high thyroid uptake, negative perchlorate discharge, low total T4 and a discrepancy between the FBI and total T4 is likely to be found in either coupling, thyroglobulin or deiodinase defects. TTie FBI measures both thyroglobulin and the iodo-tyrosines in addition to T4. Special procedures are necessary to difFerenti-ate these defects. 

Function tests of thyroid activity have little or no value in establishing the diagnosis of lymphocytic thyroiditis because of the wide spectrum of thyroid activity that may be seen in this condition. Thyroid function is likely to be normal, low normal, or subnormal, but on occasions increased function may be encountered. The diagnosis of this condition rests on a clinical suspicion of its existence, the presence of microsomal or thyroglobulin antibodies at high dilution with purposefully insensitive techniques (as discussed in Section 4.3). Where indicated, the definitive diagnosis is made by biopsy. 

However, routine thyroid studies including TSH assay and thyroid uptake of are useful in these patients to ascertain the state of the thyroid function. 

Routine studies of FBI, T4 and thyroid uptake should therefore be carried out in patients suspected of having this condition and be repeated at regular intervals (if the patient is not treated with Z-thyroxine) to follow the progress of the disease (G5). As the acute phase passes the thyroid uptake will return, and frequenfly a rebound phase is seen with increased thyroid uptake. Thyroid autoantibodies should be looked for and will be found only in a very low titer (V4) compared with the high titers found in lymphocytic thyroiditis which can sometimes mimic subacute thyroiditis if it presents acutely. Followup studies of total T4 and thyroid uptake, together with TSH assays, are desirable as some patients gradually go into a state of permanent thyroid failure. 

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