Changes in basal and stimulated TSH and other parameters of thyroid function in acromegaly after transsphenoidal surgery

Abstract

Abstract. T4 and T3 levels, TSH response to TRH and somatomedin-C levels in 63 patients with acromegaly, were measured before transsphenoidal surgery and during a 4-year follow-up period. Criteria for cure were: mean GH level <5 mU/l, suppression of GH by oral glucose tolerance test below 2.5 mU/l and normalization of paradoxical GH reaction to TRH. Nine patients underwent radioiodine studies to assess the renal and thyroid clearance of iodide, plasma inorganic iodine level and absolute iodine uptake. Among the patients 40% had goitre, with a male preponderance. T4 and T3 levels were in the normal range both before and after surgery. A transient decrease in T3 levels was found in the immediate postoperative period. Before treatment a diminished or absent TSH response to TRH was exhibited by 64% of the goitre patients and 34% of the non-goitre groups (p < 0.05). Despite normalization of GH and somatomedin-C levels and normal T4 and T3 levels no improvement of the TSH response was found during followup. No correlation between the incremental response of TSH to TRH and circulating T4 or T3 levels, basal TSH, GH or tumour size was found. There was, however, a negative correlation (r = −0.765, p < 0.05) between the incremental TSH response to TRH and somatomedin-C levels for females with goitre. Somatomedin-C levels were higher in patients with goitre than in those without goitre (95 ± 26 vs 75 ± 30 nmol/l; mean ± sd, p = 0.05). Radioiodine studies showed an increased renal clearance of iodide which was related to the increase in creatinine clearance. The absolute iodine uptake was significantly higher for male acromegalic patients than for controls (7.2 ± 2.2 vs 3.9 ± 2.3, p < 0.05) and decreased significantly postoperatively. From this study we conclude that the increased incidence of goitre in acromegaly is not caused by iodine deficiency, but is probably related to a stimulatory effect of GH or somatomedin-C on thyroid growth and function. In contrast to patients with other pituitary tumours, the impaired TSH response in acromegalic patients is not associated with hypothyroidism, and the TSH response to TRH does not normalize postoperatively, despite normalization of GH levels.