Uitrasensltlve Bioassay of Thyrotropin,

Radioimmunoassay of thyrotropin (TSH) is well-established, and highly sensitive radioimmunometric assay is often used clinically (1). Some recent results indicate that TSH immunoreactivity and bioactivity are not always synonymous (2). The detection limit of present bioassays for bovine TSH is about 0.5-1 milli-int. unit/L (3-6). More-sensitive bioassay is necessary for evaluating human TSH. The diterpene forskolin, which activates hormone-sensitive adenylate cyclase by acting directly at the catalytic subunit of enzyme, potentiates hormonal agonists that act through GTP-binding protein (Cs) (7). Low concentrations of forskolin also potentiate the cAMP response of thyroid cells to TSH (7, 8); the detection limit ofan assay for bovine __________________ ‘lUSH is about 10 milli-int. units/L (8). Here, we used this