Hyperprolactinaemia can occur in patients with primary hypothyroidism. Although its prevalence in overt hypothyroidism varies from 0 to 40%, its prevalence and clinical significance in subclinical hypothyroidism has not been studied. In this prospective, observational study, serum levels of prolactin (PRL) were measured in 167 consecutive patients presenting to our endocrinology clinic for evaluation of hypothyroidism, and correlation of PRL levels with the severity of hypothyroidism (overt or subclinical) was performed. Forty three patients (37 women, 6 men, mean age 46.18±12.98 years) had overt hypothyroidism. One hundred twenty four patients (112 women, 12 men, mean age 44.14±12.19 years) had subclinical hypothyroidism. The other potential causes of the PRL elevation were evaluated. Serum levels of thyrotropin (TSH), free thyroxine, free triiodothyronine and PRL were measured in all patients before L-thyroxine treatment and after TSH normalization. PRL elevation was found in 10 patients (23.25%) with overt hypothyroidism, and in 29 patients (23.39%) with subclinical hypothyroidism. PRL levels decreased to normal levels after thyroid function normalised with L-thyroxin treatment. In both overt and subclinical hypothyroid patients, no relationship was found between TSH and PRL levels. In conclusion, our data confirm that in overt and subclinical hypothyroidism PRL regulation is altered, and that PRL levels return to normal with appropriate L-thyroxine treatment.