Relationship of serum T4 and T3 to TSH in primary hypothyroidism.

The relationship of serum thyroxine (T4) and 3,5,3'-triiodothyronine (T3) to the thyroid-stimulating hormone (TSH) response to protirelin was studied in 34 hypothyroid patients on a stable maintenance dose of oral levo-thyroxine. Release of TSH from the anterior pituitary, as measured by basal TSH and the TSH response to protirelin, correlated better with serum T4 than serum T3. Our study supports the concept that, in treated hypothyroid patients, serum T4 has a greater relative importance in mediating the actions of thyroid hormone on the anterior pituitary than serum T3.