The thyrotrophin receptor in human thyroid plasma membranes: effect of serum immunoglobulins

A homologous receptor assay system using human thyroid membranes and 125I-labelled human TSH (hTSH) was used to study the effect of serum and serum fractions on the binding of [125I]hTSH to the membranes. Scatchard analysis showed a single population of binding sites for TSH. Gamma globulin fractions prepared from sera of patients with Graves' disease were able to displace [125I]hTSH from the membrane to a greater extent than normal gamma globulin in 21 out of 45 cases. Increased displacement activity was seen in patients with thyroiditis, hot nodules and euthyroid eye disease but not in patients with toxic multinodular goitres. Further fractionation of the gamma globulin fraction showed that the stimulatory activity was not confined to the IgG fraction. Scatchard plots showed gamma globulin fractions decreased the number of receptor sites available for TSH binding but did not alter the affinity of the receptor for TSH. IgG fractions showed different slopes and intercepts and appeared to decrease the affinity of the receptor for TSH. LATS activity in human serum may be explained on the basis of these observations on the properties of the TSH receptor.