Human Thyrotropin Receptors are Expressed Independently of the State of Thyroid Hormone Production in Thyroid Tissue

Abstract

The binding of highly purified ¹²⁵I-labelled bovine thyrotropin to preparations of crude human thyroid plasma membranes is reported. Thyroid tissues were obtained from ten patients with benign, non-functioning nodules and from three patients with benign, functioning nodules. In all cases membranes were also prepared from surrounding normal thyroid tissues. Scatchard analysis of equilibrium binding data derived from incubations with membranes from non-functioning, functioning and normal tissues all displayed a single thyrotropic binding site with the same high affinity, K_a = 2.0 - 2.3 × 10¹⁰ M^-1. This indicates that the TSH receptors present in benign, malfunctioning thyroid tissue have normal binding properties. Differences in the amount of ¹²⁵I-TSH binding to membranes from pathological and corresponding normal tissues were found when specimens from an individual patient were compared. However, such differences were not systematic between patients. In fact, the various groups of membranes from both pathological and normal tissue had similar TSH binding and, expressed as picogram labelled TSH bound per mg membrane protein (mean ± SEM) the following values were obtained: benign, non-functioning tissue 22.9 ± 2.0; benign, functioning tissue 23.4 ± 5.1; and normal tissue 23.2 ± 1.5.

The data indicate that human TSH receptors are expressed independently of the state of thyroid hormone production. It is suggested that the defective regulation of hormone synthesis in benign, malfunctioning thyroid tissue is caused by events beyond the thyrotropin receptor level.

Key words

Human Thyrotropin Receptors - Thyroid Hormon Production - Thyroid Tissue