The aims of this study were to: 1) determine the prevalence of hyperprolactinaemia in patients with newly diagnosed subclinical and overt hypothyroidism, and 2) investigate the change in PRL levels with treatment. In this observational study, patients with a new diagnosis of hypothyroidism in our endocrinology clinic were approached for participation, as were healthy controls. Patients with medical reasons for having elevated PRL levels, lactating and pregnant women were excluded from the study. No patient had kidney or liver disease. After examination to determine if clinical causes of PRL elevation were present, serum levels of thyrotropin (TSH), free thyroxine, free triiodothyronine and PRL were measured and correlation of PRL levels with the severity of hypothyroidism (overt or subclinical) was performed. Fifty-three patients (45 women, 8 men, mean age 45.3 ± 12.2 years) had overt hypothyroidism. One hundred forty-seven patients (131 women, 16 men, mean age 42.9 ± 12.6 years) had subclinical hypothyroidism. One hundred healthy persons (85 women, 15 men, mean age 43.9 ± 11.4 years) participated as controls. The same blood tests were repeated in patients after normalization of TSH levels with L-thyroxine treatment. PRL elevation was found in 36% of patients with overt hypothyroidism, and in 22% of patients with subclinical hypothyroidism. PRL levels decreased to normal in all patients after thyroid functions normalized with L-thyroxine treatment. In the hypothyroid patients (overt and subclinical) a positive correlation was found between TSH and PRL levels (r=0.208, p=0.003). PRL regulation is altered in overt and subclinical hypothyroidism, and PRL levels normalize with appropriate L-thyroxine treatment.