IS, MB, AK: nothing to disclose, JCP: consultant Interlace Medical. For hypothyroid women attempting pregnancy the optimal preconception TSH level is considered to be <2.5mIU/L, however controversy exists in regards to the upper limit of the TSH euthyroid range in women undergoing fertility treatments. To compare the outcome of OI/IUI cycles with a preconception TSH level <2.5mIU/L to that of cycles with TSH ≥2.5mIU/L (but <5.0mIU/L). Design: Retrospective study. Setting: Academic fertility center. Patients: Women with a TSH<5.0mIU/L undergoing OI with either clomiphene citrate (CC) or gonadotropins (FSH). Interventions: Data from 4542 OI cycles (28% CC/IUI, 72% FSH/IUI) were reviewed and stratified by the preconception TSH level (<2.5 vs ≥2.5mIU/L, groups 1 and 2 respectively). Primary outcome measures: clinical pregnancy, spontaneous abortion, multiple pregnancy rates per cycle (CPR, SABR, MPR respectively). Secondary outcome measures: total dose of gonadotropins, estradiol (E2) the day prior to and the day of HCG trigger (available in FSH/IUI cycles only); HCG trigger day, number of preovulatory follicles (≥13 mm), and endometrial thickness (available in all cycles). Statistics: t-test and x2 used as appropriate. In 3357 (74%) cycles the TSH was <2.5mIU/L, and in 1185 (26%) it was ≥2.5mIU/L. 368 (8.1%) patients were euthyroid, albeit on thyroid medication. 175 (14.7%) group 2 patients had a TSH ≥4.0mIU/L. Age and basal FSH were comparable among groups. Total dose of gonadotropins, peak E2 levels, pre-ovulatory follicle count, HCG trigger day, and endometrial thickness were comparable among groups. Overall, clinical pregnancy and multiple pregnancy rates did not differ among groups, however an unexpected significantly higher SAB rate was noted in women with a TSH<2.5mIU/L (Table 2). The difference was noted in FSH/IUI cycles but not in CC/IUI ones (SABR: 22.8% vs 12.1%, P<0.01, and 15.9% vs 9.7%, P:0.56, respectively). Results did not change after excluding patients with PCOS and women on thyroid medication (SABR: 22.9% vs 9.6%, P<0.01 for group 1 vs 2, respectively). Furthermore, the difference persisted despite the FSH/IUI sub-groups being comparable in age, BMI, and baseline FSH (35.8±4.4 vs 35.7±4.4 years, 24.4±4.7 vs 24.6±4.9 kg/m2, 7.9±3.5 vs 8.1±3.3 IU/L, P>0.05 for all comparisons). The application of a stricter TSH cut-off value (<2.5mIU/L) among women undergoing OI/IUI with a euthyroid TSH <5.0mIU/L does not seem to improve the outcome of the cycle. When comparing FSH/IUI cycles with a TSH<2.5mIU/L to those with a TSH≥2.5mIU/L, an unexpected higher SAB rate was noted in the former and persisted even after considering possible confounders.