Introduction: The regulation of the selection, growth and ovulation of the dominant follicle is a complex process that depends on a subtle balance of the expression of hormones and growth factors and plays a substantial role in the quality of the oocytes. The role of the thyroid axis in this process has not been thoroughly studied, despite the great evidence that highlights its importance in natural fertility. The objective of the present study was to evaluate the possible correlations between hormones in ovarian follicular fluid (FF) with oocyte maturation in women recruited for an assisted fertilization procedure. Subjects and methods: Retrospective cohort study. Fifty one women (29 to 42 years) without endocrine diseases, autoimmunity or medication affecting thyroid function were evaluated after a controlled ovarian stimulation protocol. FF was sampled by transvaginal ultrasound-guided aspiration of the hyperstimulated ovary; each follicle was individually aspirated and collected. The remnant FF collected was thawed and T3 (T3f), T4 (T4f), free T4 (fT4f), LH (LHf) and FSH (FSHf) were measured using chemiluminescence immunoassay by Advia Centaur XP autoanalizer. All oocytes retrieved were evaluated to analyze the complex cumulus corona expansion degree and the oocytes maturational stage was determinated after denudation of oocytes by enzymatic and mechanical methods. Only those with a visible polar body were classified as mature or metaphase II (MII). The oocyte maturation rate (OMR) was calculated as: Number of metaphase II oocytes / Number of oocytes retrieved x 100. Correlations between parameters were analyzed by Spearman test. Results: hormone levels in FF were: T3f: 99.1 ± 29.7 ng/dl, T4f 6.9 ± 2.42 ug/dl, fT4f: 1.2 ± 0.2 ng/dl, FSHf: 7.6 ± 3.3 mUI/ml and LHf: 0.63 (0.42 – 1.63) mUI/ml. The OMR was 66 (57-74) % and the median
number of oocytes retrieved was 5 (0-18). T4f showed a tendency to correlate with the number of oocytes retrieved (r=0,274, p=0,052) and the number of metaphase II oocytes (r = 0,270, p = 0,055). The OMR correlated only with LHf levels (r=0,369, p=0,009). FSHf showed a positive correlation with LHf and fT4f (r=0.369, p=0.009; r=0,338; p=0,018, respectively) and showed a tendency to correlate with T4f (r=0,267; p=0,061). Conclusion: The tendency of T4 in FF to correlate with the number of oocytes recovered and with the number of mature oocytes, as well as the correlation found between fT4F with follicular fluid FSH levels, suggest a facilitating role of thyroid hormones in follicular development and oocyte maturation. However, more studies with larger number of patients should be conducted to confirm these results.