Low TSH Is Associated With Frailty in an Older Veteran Population Independent of Other Thyroid Function Tests

Low TSH is associated with frailty in the older adult. We studied whether low TSH is an independent marker of frailty or is an indicator of subclinical hyperthyroidism, which in turn predicts frailty. Of outpatient veterans seen between January 2005 and December 2016, we identified 100 patients aged ≥60 years with two low TSH (<0.5 µIU/ml) and one fT3 measurement and 50 matched controls (TSH 0.5-5.0 µIU/ml). We used a deficit accumulation approach to create a frailty index (FI). The higher the FI, the more likely (p<0.001) that patients had expired. Patients with low (0.31 ± 0.11 µIU/mL) versus normal (1.84 ± 0.84 µIU/mL) TSH had higher mean FI compared to controls (0.25 ± 0.12 vs. 0.15 ± 0.07, p < .001). Low TSH was significantly associated with frailty (p < .001), independent of age. However, lower TSH was not associated with higher fT3 or fT4 levels. There was a nonsignificant inverse association of fT3 levels with FI (p = .13), which disappeared when adjusted for age. Similar to prior studies, low TSH was associated with frailty. However, neither fT3 nor fT4 predicted low TSH or FI, suggesting that the association of low TSH with frailty is not due to subclinical hyperthyroidism, but perhaps to effects of comorbidities on TSH secretion.