The Pituitary-Thyroid Axis in Acromegaly

Summary

The pituitary-thyroid axis of 12 acromegalic patients was evaluated by measurement of the serum concentrations (total and free) of thyroxine (T₄), triiodothyronine (T₃) and reverse T₃ (rT₃) and thyrotropin (TSH), growth hormone (GH) and prolactin (PRL) before and after iv stimulation with thyrotropin releasing hormone (TRH). Using an ultrasensitive method of TSH measurement (IRMA) basal serum TSH levels of the patients (0.76, 0.07-1.90 mlU/l) were found slightly, but significantly (P < 0.01), lower than in 40 healthy controls (1.40, 0.41-2.50 mlU/l). The total T₄ levels (TT₄) were also reduced (84,69-106 nmol/l vs 100, 72-156 nmol/l, P < 0.01) and significantly correlated (P < 0.02, R=0.69) to the TSH response to TRH, suggesting a slight central hypothyroidism. The acromegalics had, however, normal serum levels of TT₃ (1.79, 1.23-2.52 nmol/l vs 1.74, 0.78-2.84 nmol/l, P > 0.10), but significantly decreased levels of TrT₃ (0.173, 0.077-0.430 nmol/l vs 0.368, 0.154-0.584 nmol/l, P < 0.01) compared to the controls. The serum concentration of the free iodothyronines (FT₄, FT₃, FrT₃) showed similar differences between acromegalics and normal controls. All the acromegalics showed a rise of serum TSH, GH and PRL after TRH. Positive correlation (P < 0.05, R=0.59) was found between the TSH and GH responses, but not between these two parameters and the PRL response to TRH.

These findings may be explained by the existence of a central suppression of the TSH and GH secretion in acromegalic subjects, possibly exerted by somatostatin. Euthyroidism might be maintained by an increased extrathyroidal conversion of T₄ to T₃.

Key-Words

TSH - Growth Hormone - Prolactin - Somatostatin - Thyroid Hormones