The Pituitary-Thyroid Axis in Acromegaly

The pituitary-Thyroid axis of 12 Acromegalic patients was evaluated by measurement of the concentration (Total and free of thyroxine T4, T3 and revers T3 (rT3) and TSH, GH and prolactin before and after in stimulation with TRH. Using an ultra­sensitive method of TSH measurement (IRMA) basal serum TSH levels of patients (0.76, 0.07-l.90mlU/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<0.01) compared to the controls. The serum concentrations of the (FT4, FT3, FrT3) showed similar differences between acromegalics and normal controls. All the acromegalics showed a rise of serum TSH, GH and PR, after TRH. Positive correlation (P<0.05, R=0.59) was found between the TSH response, 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 mained by and increase extra thyroid convertion of T4 to T3.