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Subclinical Hypothyroidism and Cardiovascular Risk – An Overview of Current Understanding

European Endocrinology, 2011;7(1):53-7 DOI: http://doi.org/10.17925/EE.2011.07.01.53

Abstract:

Abstract
Subclinical or mild hypothyroidism is often associated with adverse cardiovascular risk factors, such as high cholesterol, together with hypertension, endothelial dysfunction and other atherosclerotic cardiovascular risk factors. The ischaemic abnormalities are probably related to long-term consequences of a slowly progressing development of hypothyroidism. In recent years, it has become evident that a consensus on the exact limits for cut-off between normal and subclinically hypothyroid individuals is not currently possible. The main reasons for this are differences for measurement of serum thyroid-stimulating hormone (TSH), that reference populations are very different and that a person’s intra-individual variability is much narrower than any population-based interval. Finally, the prevalence of subclinical hypothyroidism varies from 4 to 17% in different normal populations. Available evidence indicates that patients with subclinical hypothyroidism have developed or are at risk of developing an adverse cardiovascular profile and subclinical hypothyroidism is most likely a mild variant of overt hypothyroidism. There is currently no evidence for a treatment benefit, but studies to demonstrate the expected minor improvements have not been performed on a sufficiently large scale. Patients should be informed about the disease and based on a combined clinical and laboratory judgement, should be offered a therapeutic trial in case of even vague symptoms.

Keywords
Subclinical hypothyroidism, thyroid-stimulating hormone (TSH), decision level, reference population, cardiovascular, mortality, ischaemic heart disease, thyroid hormone

Disclosure: The author has no conflicts of interest to declare.
Received: 28 August 2010 Accepted: 1 October 2010 Citation: European Endocrinology, 2011;7(1):53–7 Correspondence: Ulla Feldt-Rasmussen, Department of Medical Endocrinology PE 2132, Rigshospitalet, National University Hospital, Blegdamsvej 9, DK-2100 Copenhagen, Denmark. E: ufeldt@rh.dk
Keywords: Subclinical hypothyroidism, thyroid-stimulating hormone (TSH), decision level, reference population, cardiovascular, mortality, ischaemic heart disease, thyroid hormone
Disclosure: The author has no conflicts of interest to declare.
Received: August 28, 2010 Accepted: October 01, 2010
Correspondence: Ulla Feldt-Rasmussen, Department of Medical Endocrinology PE 2132, Rigshospitalet, National University Hospital, Blegdamsvej 9, DK-2100 Copenhagen, Denmark. E: ufeldt@rh.dk

Subclinical hypothyroidism comprises elevated serum thyroidstimulating hormone (TSH) in the context of normal peripheral thyroid hormone concentrations.1 Although this definition is highly dependant on methods for TSH measurement and the definition of the upper limit of normal reference values, many publications have come out in recent years concerning the long-term consequences of subclinical hypothyroidism and guidelines on its management have even been presented.2

Overt hypothyroidism is associated with abnormal haemodynamics resulting in a reduced cardiac index and an impaired renal perfusion,3 combined with adverse cardiovascular risk factors such as high total and low-density lipoprotein (LDL) cholesterol together with hypertension, endothelial dysfunction, increased central arterial stiffness and the other atherosclerotic cardiovascular risk factors. Even mild or subclinical hypothyroidism demonstrate these findings more often than do normal controls.1,2 All of the above factors may thus, in part, be responsible for a frequently described association between subclinical hypothyroidism and ischaemic heart disease. This article aims to summarise the current understanding of the cardiovascular risks in mild or subclinical hypothyroidism, while other aspects of the condition such as its effects on cognitive function, fertility aspects and risks that develop during foetal brain development will not be dealt with here.
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Keywords: Subclinical hypothyroidism, thyroid-stimulating hormone (TSH), decision level, reference population, cardiovascular, mortality, ischaemic heart disease, thyroid hormone