Both hyperthyroidism and hypothyroidism produce changes in cardiac contractility, myocardial oxygen consumption, cardiac output, blood pressure, and systemic or pulmonary vascular resistance. In almost all cases these cardiovascular changes are reversible when the underlying thyroid disorder is recognized and treated. Pulmonary hypertension (PAH) has been associated with thyroid dysfunction, but primarily with hyperthyroidism. The vast majority of patients with this form of PAH are usually older with toxic multinodular goitre. Data currently available suggest a direct influence of TH on pulmonary vasculature. Possible mechanisms include: 1) enhanced catecholamine sensitivity, causing pulmonary vasoconstriction, a reduction in pulmonary vascular compliance and an increase in vascular resistance; 2) increased metabolism of intrinsic pulmonary vasodilating substances (prostacyclin, nitric oxide); 3) decreased or impaired metabolism of vascontrictors (serotonin, endothelin 1 and tromboxane). In some cases (Graves's and Hashimoto's disease) and an autoimmune process inducing endothelial damage may play a key role. Future studies should focus on discovering the immunogenetic overlap between autoimmune thyroid diseases and PAH: common human leukocyte antigen alleles, susceptibility loci and so on. Such an understanding of the genetic and immune factors may ultimately lead to novel effective approaches in the treatment of PAH. At present, thyroid function tests should be considered in the investigation of all patients with PAH.