Low-normal thyroid function and cardiometabolic disorders

Low-normal thyroid function commonly shows up in lab results in my general practice, mostly due to the diffuse autoimmune phenomena so widespread now, but it seems to be often overlooked. A study just published in the European Journal of Clinical Investigation offers more evidence that low-normal thyroid function should be respected as a risk factor, in this case for cardiovascular and metabolic disorders. The authors state:

"Subclinical hypothyroidism may adversely affect the development of cardiovascular disease (CVD). Less is known about the role of low-normal thyroid function, that is higher thyroid-stimulating hormone and/or lower free thyroxine levels within the euthyroid ['normal'] reference range, in the development of cardio-metabolic disorders. This review is focused on the relationship of low-normal thyroid function with CVD, plasma lipids and lipoprotein function, as well as with metabolic syndrome (MetS), chronic kidney disease (CKD) and nonalcoholic fatty liver disease (NAFLD)."

The authors surveyed a range of reviews and meta-analyses derived from clinical and basic research papers, obtained published up to November 2014 and found:

"Low-normal thyroid function could adversely affect the development of (subclinical) atherosclerotic manifestations. It is likely that low-normal thyroid function relates to modest increases in plasma total cholesterol, LDL cholesterol and triglycerides, and may convey pro-atherogenic changes in lipoprotein metabolism and in HDL function. Most available data support the concept that low-normal thyroid function is associated with MetS, insulin resistance and CKD, but not with high blood pressure. Inconsistent effects of low-normal thyroid function on NAFLD have been reported so far."

See earlier posts for studies reporting additional adverse effects from low-normal thyroid and low-normal free T3. Practitioners should be alert to anti-thyroid antibodies indicating a pre-Hashimoto's state and test for iodine insufficiency (by 24 hour urine collection) when indicated. The authors conclude:

"Observational studies suggest that low-normal thyroid function may be implicated in the pathogenesis of CVD. Low-normal thyroid function could also play a role in the development of MetS, insulin resistance and CKD, but the relationship with NAFLD is uncertain."