Serum free thyrotropin subunit in congenital isolated thyrotropin deficiency.

In two patients with congenital isolated thyrotropin (TSH) deficiency, serum TSH determined by a sensitive immunoradiometric assay (IRMA) was consistently undetectable. The basal levels of serum free TSH-alpha subunit (TSH-alpha) determined by a specific radioimmunoassay (RIA) were elevated in the hypothyroid state, and decreased to the undectable level during displacement therapy with thyroid hormone. The serum free TSH-alpha significantly increased following intravenous administration of thyrotropin releasing hormone (TRH). Serum free TSH-beta subunit (TSH-beta) was undectable. These findings suggest that TSH deficiency in this disease is not due to absence of thyrotroph in the pituitary gland or deficiency of TSH-alpha, but to abnormalities of the TSH-beta gene.