Background: Abnormal uterine bleeding due to ovulatory dysfunction (AUB-O) is a common gynaecological problem in women of reproductive age. Thyroid hormones influence the hypothalamic-pituitary-ovarian axis, ovarian steroidogenesis, sex hormone-binding globulin, and endometrial response. Therefore, both overt and subclinical thyroid dysfunction may present with menstrual disturbances. Objective: To determine the association between thyroid disorders and AUB-O, compare serum thyroid-stimulating hormone (TSH) and free thyroxine (FT4) levels between cases and controls, and describe the pattern of menstrual abnormality in relation to thyroid status. Methods: This case-control study was conducted in the Department of Obstetrics and Gynaecology, Bangabandhu Sheikh Mujib Medical University (BSMMU), Dhaka, Bangladesh, from July 2015 to November 2016. Sixty (60) women aged 18-45 years with diagnosed AUB-O were compared with 60 matched controls with normal menstrual patterns. Serum TSH and FT4 were measured by chemiluminescent immunoassay. Data were analyzed and compared by statistical tests. Results: Thyroid dysfunction was significantly more frequent among cases than controls (50.0% versus 21.7%; OR 3.62, 95% CI 1.52-8.69; p= 0.001). Mean serum TSH was higher in cases than controls (12.6±10.6 versus 6.4±21.2 mIU/L; p= 0.045), and mean serum FT4 was also significantly higher (2.4±3.6 versus 1.3±0.7 ng/dl; p= 0.021). Menorrhagia was the commonest complaint (46.7%). Hypothyroidism and subclinical hypothyroidism were mainly associated with menorrhagia, whereas hyperthyroidism and subclinical hyperthyroidism were mainly associated with oligomenorrhoea. Anaemia was significantly more common in cases than controls (60.0% versus 15.0%; p= 0.001). Conclusion: Thyroid dysfunction was significantly associated with AUB-O in reproductive-age women. Routine thyroid function assessment, particularly serum TSH and FT4, should be included in the evaluation of women with AUB-O to support targeted medical treatment and reduce unnecessary hormonal or surgical intervention.