SOURCE: Grossman A, Weiss A, Koren-Morag N, et al. Subclinical thyroid disease and mortality in the elderly: A retrospective cohort study. Am J Med 2016;129:423-430.
Subclinical thyroid disease is defined as altered thyroid stimulating hormone (TSH) level in the presence of normal thyroid hormone (T4, T3) levels. Hence, subclinical hypothyroidism exists when TSH levels are elevated but T4 is normal, and subclinical hyperthyroidism exists when TSH levels are reduced, but T3 and T4 levels are normal. Since a substantial number of cases of overt hyperthyroidism and hypothyroidism are immediately preceded by a period of subclinical status, it probably should not be surprising that subclinical thyroid disease status (both hyper and hypo) is associated with mortality in elders.
Grossman et al performed a retrospective analysis on patients > 65 years of age (n = 17,440), comprised of individuals with normal thyroid status (n = 14,946), subclinical hyperthyroidism (n = 538), and subclinical hypothyroidism (n = 1,956) who were followed for as long as 10 years.
Both subclinical hypo- and hyperthyroidism were associated with increased risk for mortality (hazard ratio = 1.53 and 2.33, respectively). In the population of subjects with subclinical hypothyroidism, a TSH > 6.35 mIU/L was the threshold for increasing mortality risk. Subclinical hyperthyroidism has been associated with increased risk for atrial fibrillation, so the increased mortality risk in this population is perhaps less surprising.