Thymic Hormone Levels in Animals and Man

In this section we will summarize the status of the currently available methodologies for determining serum levels of products that reflect the endocrine function of the thymus. It had been hoped that specific abnormalities in blood levels of thymic hormones would provide important information for diagnosing specific immunodeficiency disorders. However, this has not turned out to be the case, and even though the currently available assays can frequently detect abnormalities on the part of the endocrine thymus, there have been no specific serum alterations that are diagnostic of any particular immunodeficiency disorder, and no strong correlations exist between low or high serum thymic hormone levels and the immune status of the patient as defined in T cell functional assays. 

The most widely applied serum bioassay is the murine rosette-azathio-prine assay developed by Bach and Dardenne (1972a). This is the only currently available bioassay that has generated reproducible results in laboratories around the world (Bach and Dardenne, 1972a Dardenne and Bach, 1973 Garaci et al., 1978 Burton etal., 1978 Iinaizumi etal., 1981 Iwataef 

With this assay, both natural and synthetic FTS standard preparations exhibit activity at concentrations as low as 0.6 x 10 ng/ml. Partially purified thymic preparations, such as thymosin fraction 5, are active in this assay at much higher concentrations than FTS. For example, TF5 was found to be active at final concentrations between 0.35 and 350 ng/ml (Twomey and Kouttab, 1982). Thymopoietin was active in the rosette bioassay at concentrations between 2 and 70 ng/ml (Twomey and Kouttab, 1982), whereas Taj was active at concentrations between 10and 10 M (Wong and Merrifield, 1980 Ciardelli et al., 1982). Several synthetic C-terminal Toj peptides appeared to be almost as active as FTS itself (Ciardelli et al., 

Comparative Studies with Bioassays in Human Diseases 

Serum thymic hormone levels, based on either the bioassay of Bach and Dardenne or the method of Twomey and colleagues, have now been determined for a variety of primary and secondary immunodeficiency disorders (summarized in Table 1). 

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