Increase of circulating thyroid hormones following oral administration of TRH in preclinical hyperthyroidism. Biological efficacy of minimal TSH stimulation
In patients with multinodular goiter and preclinical hyperthyroidism (euthyroid values of free T4-index, free T3-index, free T4, absent TSH response to oral TRH (40 mg), i.e. TSH after TRH less than 1.0 microU/ml) a statistical increase in the serum-T3 (free T3-index), serum-T4 (free T4-index) and free T4 was observed after oral TRH. No thyroid response to oral TRH occurred in multinodular goiter patients with hyperthyroidism or in preclinically hyperthyroid +athyrotic patients under T4. The thyroid response in patients with a preclinically hyperthyroid goiter was significantly lower than the response in goiter patients with a normal TSH reserve and in controls. The results demonstrate that (1) in euthyroid TRH-TSH unresponsiveness the pituitary TSH-reserve may not be completely suppressed; (2) very small variations in TSH not detectable by the available radioimmunologic assay may be of biological importance. In euthyroid TRH-TSH-unresponsive patients under T4 (simple goiter, differentiated thyroid carcinoma) thyrotropic function may not be completely suppressed. In euthyroid TRH-TSH-unresponsive patients with an autonomously functioning adenoma, additional exogenous suppression should be evaluated for protection of paranodular tissue before ablative radioiodine is given.