Objective The aim of this investigation was to evaluate the effects of hyperprolactinaemia on thyroid function, volume and nodularity in patients with prolactinoma.,Context Hyperprolactinaemia has been associated with various autoimmune diseases; however, the data on the correlation between the level of prolactin (PRL) and thyroid disorders have not been adequately clarified.,Design Case-control study.,Patients Forty-eight subjects with new diagnosis of hyperprolactinaemia (group 1) and 39 subjects undergoing treatment for prolactinoma (group 2) were recruited from our outpatient clinic. Fifty-two healthy subjects were included as a control group (group 3).,Measurements The serum PRL, thyroid-stimulating hormone (TSH), thyroxine (free T4), thyroidal microsome (anti-TPO) and antithyroglobulin antibodies (TgAb) levels were evaluated, and ultrasonographic thyroid volume was calculated.,Results The frequencies of positive anti-TPO and TgAb were significantly higher in group 1 than in groups 2 and 3 (P = 0.008). Also, the percentage of patients with thyroid heterogeneity were significantly higher in groups 1 and 2 than in group 3 (P < 0.05). The percentage of patients with thyroid nodules were higher in group 1 than in groups 2 and 3 (p(1-2) = 0.03, p(1-3) = 0.05 and p(2-3) = 0.637). The mean thyroid volume was significantly higher in group 1 (P = 0.001), and a positive correlation was found between thyroid volume and the level of PRL (r = 0.616; P = 0.0001). Prolactin had a significant effect on the total volume according to stepwise multiple linear regression analysis (adjusted R-2 is 0.268; P < 0.0001).,Conclusions Patients with hyperprolactinaemia have significantly increased thyroid volume, thyroid autoimmunity and nodule prevalence.