Cathepsin B (CB) is involved in the hydrolysis of thyroglobulin (Tg) and thought to be regulated by thyroid stimulating hormone (TSH) in the normal thyroid. Our analyses of 91 thyroid tissues from 71 patients with Graves' disease (GD), multinodular goiter (MNG), papillary carcinoma (PC), or follicular carcinoma (FC), demonstrated a 2-fold increase in expression of CB in GD and an average increase of 1.5-fold in MNG (varying from 10-fold below normal to 6-fold above normal in MNG nodules), as might be predicted by the altered functional status of thyroid follicular cells in those diseases. However, CB activity was not downregulated in conjunction with the known "blocking effect" of malignancy on many thyroid functions, but rather increased on average 9-fold in papillary carcinomas (n = 33), and also showed a marked increase in 2 follicular carcinomas. Activity measurements were confirmed by CB protein detection on Western blot with moderately increased CB protein levels demonstrated in GD, variable expression in nodules of MNG, and markedly increased protein expression in carcinomas. In all diseased states, increased protein was detected primarily as overexpression of the 27 kd heavy chain of 2-chain mature CB and less frequently as overexpression of 31 kd single-chain mature CB. However, an additional 35 kd protein form was noted in 3 of 9 PCs, 1 of 2 FCs, and 1 of 4 GD cases but in none of 10 MNG cases. In conjunction with elevated CB activity plus additional protein bands on Western blots, altered patterns of CB immunohistochemical staining were observed, irrespective of the type of thyroid disease, suggesting certain common changes in CB expression, posttranslational processing, and vesicular trafficking. In summary, GD and MNG thyroid tissues demonstrated altered CB expression in keeping with predicted functional changes in thyroid follicular cells, while increased CB expression in carcinomas indicated a more pathological role for CB in thyroid cancers, possibly related to the processes of invasion or metastasis.