Thyrotrophin receptors in normal and neoplastic (primary and metastatic) canine thyroid tissue

Thyrotrophin (TSH) is the conditional growth factor of thyroid epithelial cells. Abnormalities in TSH-receptor binding such as a low receptor number or low binding affinity may be a marker of thyroid carcinoma or metastases, or may exhibit a relationship with the functional variability of such tissues. The dog was used as a model to characterize TSH-receptor binding in normal thyroid tissues, naturally occurring thyroid neoplasms and distant metastases. In normal dog thyroid tissues, specific 125I-labelled TSH binding ranged from 2.7 to 15.5%, and low cross-reactivity with bovine LH (0.023%) was observed. One class of TSH-binding sites was found in eight normal thyroid tissues and 22 thyroid carcinomas; two normal thyroid tissues and one tumour exhibited two classes of binding sites. The concentration of binding sites was lower in the five carcinomas with reduced pertechnetate uptake (0.09 pmol/mg protein) than in the five thyroid neoplasms with increased uptake (0.19 pmol/mg) (P = 0.055). Compared with the original carcinoma tissues, TSH binding revealed a reduced binding affinity in eight out of eleven metastases. Two metastases showed a complete absence of TSH binding, suggesting that they were not dependent on TSH for growth. We conclude that one class of TSH-binding site is predominant in normal dog thyroid tissues and dog thyroid carcinomas. TSH could therefore contribute, at least in theory, to further growth of primary dog thyroid carcinomas. Secondly, assays measuring TSH binding may not be able to discriminate between malignant and benign dog thyroid tumours. TSH receptor number or affinity may be related to the functional variability of thyroid neoplasms.(ABSTRACT TRUNCATED AT 250 WORDS)