In elderly individuals with low selenium status, plasma selenium concentration below 80 µg/L is associated with elevated thyroid-stimulating hormone (TSH) and reduced free triiodothyronine (fT3), indicating suboptimal thyroid hormone metabolism in the absence of overt thyroid disease.

When selenium levels are low, the enzymes that convert the inactive thyroid hormone T4 into the active form T3 do not work properly. This causes less active T3 to be available in the blood, so the brain detects low thyroid activity and signals the thyroid gland to produce more TSH to try to fix it. At the same time, the body shifts some T4 into an inactive form called reverse T3, which further reduces the amount of active hormone available.