Postoperative evaluation of patients with acromegaly: Clinical significance and timing of oral glucose tolerance testing and measurement of (free) insulin-like growth factor I, acid-labile subunit, and growth hormone-binding protein levels

Context: It is not exactly known when patients with acromegaly should be evaluated for cure after transsphenoidal adenomectomy (TA). Objective: The objective of this study was to define the optimal time point of postoperative evaluation by serial measurements of glucose-suppressed GH levels [oral glucose tolerance test (OGTT)] and the GH-dependent parameters IGF-I, free IGF-I, acid labile subunit (ALS), and GH-binding protein (GHBP). Design: We describe a prospective study with 1-yr follow-up. Setting: The study was conducted at a university hospital. Patients: Seventeen patients with acromegaly were included in the study. Main Outcome Measures: The main outcome measures were OGTT results at 1, 2, 3, 8, and 12 wk after TA; weekly measured GH, (free) IGF-I, ALS, and GHBP levels up to 12 wk; and total IGF-I levels measured at 52 wk. Results: Postoperatively, nine patients were in remission with an OGTT GH nadir of less than 0.5 mu g/liter and normalized IGF-I levels, whereas eight patients had persistent acromegaly. In both cured and noncured patients, OGTT results at 1 wk after TA were highly reproducible over time. In contrast, early postoperative IGF-I levels fluctuated and only stabilized at 12 wk. In all cured patients, free IGF-I levels rapidly normalized within 2 wk after TA (specificity, 100%). Preoperative ALS levels were elevated in all patients and normalized only in the cured patients after TA (specificity, 89%). Preoperative GHBP levels were low and increased from 2 wk after surgery. Conclusions: We show that in the postoperative evaluation of patients with acromegaly, already 1 wk after surgery, an OGTT using 0.5 mu g as the GH nadir cutoff value has a high predictive value for cure, whereas early IGF-I levels show varying patterns toward stabilization. Therefore, IGF-I should be measured as a predictive parameter not within 3 months after surgery. Free IGF-I and ALS levels may have an additional value in the postoperative assessment of disease activity.