ACUTE AND CHRONIC EFFECTS OF OCTREOTIDE ON THYROID AXIS IN GROWTH HORMONE-SECRETING AND CLINICALLY NONFUNCTIONING PITUITARY-ADENOMAS

The effect of somatostatin on thyroid function was studied in 12 patie nts with growth hormone (GH)secreting and eight patients with clinical ly non-functioning adenomas (NFA) and normal pituitary/thyroid axis; t he patients were subjected to the administration of octreotide (OCT), which is a long-acting somatostatin analog. All the patients received an acute test with 100 mu g of OCT, both short term (1 month) and long term (6 months), with doses ranging from 300 to 600 mu g/day. Serum t hyroxine (T-4), triiodothyronine (T-3), free T-4, free T-3, thyroglobu lin and basal and thyrotropin (TSH)-releasing hormone (TRH)-stimulated TSH were evaluated before and after 1 and 6 months of therapy. Circul ating GH and insulin-like growth-factor I (IGF-I) in acromegalics and GH, IGF-I and a-subunit in NFA were assessed at baseline and every mon th. The acute administration of 100 mu g of OCT significantly reduced the TSH response to TRH (p < 0.01) in both acromegalics and NFA. In al l the patients OCT administration caused a significant decrease of GH, IGF-I and alpha-subunit levels (p ( 0.01). In addition, after 1 month of therapy both baseline and TRH-induced TSH secretion were decreased significantly in acromegalics and NFA. After 6 months of therapy, bas eline and TRH-induced TSH was still reduced in NFA. Conversely, in acr omegalics, baseline TSH levels were increased while TSH response to TR H was inhibited. No change of T-4, T-3, free T-4 and free Tg was obser ved in NFA, whereas a slight but significant increase of T-4 and decre ase of T-3 was recorded in acromegalics. In conclusion, OCT does seem to possess long-term suppressive effects on TSH response to TRH, both in acromegalics and NFA. The lack of basal TSH level inhibition in acr omegalics could depend on the restored peripheral conversion of T-4 in to Tg due to the normalized GH levels during long-term OCT administrat ion.