Chapter 109 Hypertension and Hypothyroidism: A thyroid dysfunction frequently associated with an abnormal dietary iodine intake

Abstract Hypertension is a very common disorder. One-third of the United States population have hypertension or are taking anti-hypertensive medications. In addition, 45 million adults in the United States (20% of total population) suffer from pre-hypertension. Hypertension cannot be classified solely by discrete blood pressure thresholds. An important emerging concept is that hypertension is frequently associated with additional co-morbidities that contribute to increasing cardiovascular risk. Actually, hypertension represents a major risk factor for vascular, cardiac, renal and cerebral pathology so that the existence of so- called target organ damage (in vessels, heart, kidney and brain) is a criterion for assessing the clinical severity of the disease. Hypothyroidism represents the most common thyroid function disorder; worldwide, the prevalence of hypothyroidism varies by iodine intake and clearly increases with age, by reaching up to 20% in women older than 60 years. The foremost cause of congenital hypothyroidism remains endemic iodine deficiency and in adults chronic autoimmune (Hashimoto) thyroiditis that is more common in geographic areas of higher dietary iodine. The presence of an association between hypothyroidism and hypertension is an acquisition that has been increasingly confirmed and validated over the time. Results rising from the bulk of the studies suggest a convincing association between hypertension and hypothyroidism, with a prevalence of hypertension in hypothyroid subjects, particularly of elevated diastolic blood pressure nearly triple than that seen in the general population. In subjects over 50 years of age, overall data indicate a 30% prevalence of hypertension in hypothyroid subjects. Moreover, population-based studies revealed the presence of an unrecognized hypothyroidism in 3-5% of untreated hypertensive patients and indicate the existence of a continuous linear relationship between thyroid function and cardiovascular risk regarding what concerns both the atherogenic and metabolic profiles. However, the observed association between hypothyroidism and hypertension does not prove a causal relationship. Considerable further exploration is needed regarding on mechanisms of disease and in particular on the relationships between a hypothyroid state and other (neuro-) endocrine systems that may determine and worsen a hypertensive state. At the same time, new acquisition is necessary to increase our understanding of the role of thyroid hormone deficiency in target organ damage induced by hypertension.