Iodination of thyroid-stimulating hormone for receptor-binding studies with human thyroid membranes: effects of specific activity and method of iodination.

Three different methods were compared for 125I-labelling of thyroid-stimulating hormone (TSH) for use in receptor-binding studies with human thyroid membranes: these were the chloramine-T, Bolton-Hunter and lactoperoxidase methods. Chloramine-T proved to be an inferior method to the other two. Iodinations to different specific activity (0.7--9.4 Bq/pg) were also compared: too high a specific activity led to reduced binding and a dramatic shift in the pH optimum for the TSH-receptor interaction. A specific acitivity of 2.5 Bq/pg should not be exceeded if binding of 125I-labelled TSH is to be representative of the binding of the natural hormone. Under these conditions, pH 7.5 was optimal for binding of TSH to its receptor. Repurification of the labelled TSH by receptor adsorption also proved to be essential.