Subclinical thyroid dysfunction and cardiovascular consequences: An alarming wake-up call?

Subclinical thyroid dysfunction (STD), presenting as subclinical hypothyroidism (SHypo) or subclinical hyperthyroidism (SHyper), defined as abnormal serum thyrotropin (TSH) and normal free thyroid hormones, is associated with increased cardiovascular (CV) risk and mortality. Depending on the degree of such dysfunction, atherosclerosis, coronary artery disease, heart failure and cardiac arrhythmias, predominantly atrial fibrillation, characterize both disorders and increase CV and total mortality compared to euthyroid persons. There are some differences in the mechanisms involved in the increased CV risk incurred by each type of STD, with more traditional CV risk factors clustered in SHypo than in SHyper, while the role of the TSH or its absence thereof, together with the respective, even subtle, changes incurred in thyroid hormone concentrations, seem to adversely influence the CV system in both types of STD. There is evidence that treatment of STD confers potential benefits by reducing CV events, however, no consensus has been reached due to lack of randomized controlled studies. Nevertheless, due to accumulating evidence from observational studies, many authorities agree that individuals with severe SHypo (TSH > 10 mIU/L) or grade 2 SHyper (TSH < 0.1 mIU/L) should receive treatment, mostly for the increased risk of CV morbidity and mortality. The evidence reviewed herein should alert and help the clinician to wake up to these two potentially alarming conditions of STD as they may confer serious CV complications, while their treatment appears quite beneficial.