The Pituitary-Thyroid Axis in Patients with Pituitary Disorders

Summary

The pituitary-thyroid axis of 12 patients, exposed to transsphenoidal pituitary microsurgery because of nonfunctioning adenomas (6), prolactinomas (3) and craniopharyngioma (1), or to major pituitary injury (1 apoplexy, 1 accidental injury), was controlled more than 6 months following the incidents. The patients did not receive thyroid replacement therapy and were evaluated by measurement of the serum concentration of thyroxine (T₄), 3,5,3'-triiodothyronine (T₃), 3,3',5'-triiodothyronine (rT₃), T₃-resin uptake test and thyrotropin (TSH, IRMA method) before and after 200 μg thyrotropin releasing hormone (TRH) iv.The examination also included measurement of prolactin (PRL) and cortisol (C) in serum. Apart from 1 patient with pituitary apoplexy all had normal basal TSH levels and 9 showed a significant TSH response to TRH. Compared to 40 normal control subjects the 12 patients had significantly decreased levels of T₄, T₃ and rT₃ (expressed in free indices), while the TSH levels showed no change. Five of the patients, studied before and following surgery, had all decreased and subnormal FT₄I (free T4 index) after surgery, but unchanged FT₃I and TSH. The levels of FT₄I were positively correlated to both those of FT₃I and FrT₃I, but not to TSH. The TSH and thyroid hormone values showed no relationship to the levels of PRL or C of the patients exposed to surgery.

It is concluded that the risk of hypothyroidism in patients exposed to pituitary microsurgery is not appearing from the TSH response to TRH, but from the thyroid hormone levels.

Key-Words

Thyrotropin - Thyroxine - Triiodothyronine - Pituitary Neoplasms - Hypophysectomy