BACKGROUND Thyroid hormones have profound effects on reproduction and pregnancy. Normal thyroid function is necessary to maintain reproductive physiology. An abnormality in its function may be a cause of infertility, manifesting as abnormal menstrual cycles. AIM To determine the serum levels of thyroid hormones and menstrual patterns among infertile and fertile women at UITH. OBJECTIVES The objectives of the study were: to determine and compare the serum levels of thyroid hormones in infertile women attending gynaecology clinic of UITH and the fertile control group; to determine and compare the prevalence of thyroid dysfunction among the infertile and fertile control groups; to compare menstrual patterns in infertile women and the fertile control group, and finally to determine the association between thyroid abnormalities and the menstrual patterns in infertile and fertile women. METHODS The study was carried out at the gynaecology clinics of the Department of Obstetrics and Gynaecology and the Family Planning Clinic of University of Ilorin Teaching Hospital. The study was a comparative cross sectional study carried out on patients in the reproductive age group who presented with history suggestive of either primary or secondary infertility. Controls were consenting women of the reproductive age group who were new clients at the family planning clinic with no history of infertility. The study was based on purposive sampling technique and a study proforma was administered to collect data. Venous blood samples for thyroid hormones levels (fT3, fT4 and TSH) were collected and measured using ELISA technique. One hundred and six women were recruited for this study, fifty-three women in each arm of study