Low TSH Levels in Nursing Home Residents Not Taking Thyroid Hormone

Abstract

Many practitioners perform a thyroid stimulating hormone (TSH) assay as a screening test in older patients. The introduction of sensitive TSH assays with lower normal limits has created a laboratory abnormality that is often of uncertain significance. Mechanisms include autonomous overproduction of thyroid hormone, nonthyroidal illness including medication effects, and hypothalamic/pituitary dysfunction. To characterize the clinical status and course of nursing home residents with low TSH and normal total T4 levels in the absence of treatment with thyroid hormone. Retrospective chart review was performed to determine participants status at the time of low TSH level, with additional recording of follow-up thyroid hormone levels, cardiac rhythm, and mortality. Mortality was compared with that of a control group matched for age and sex. A nursing home for veterans and their spouses. Forty subjects with low TSH and initially normal total T4 were identified. Only three subjects were subsequently diagnosed as hyperthyroid. TSH levels of 18 subjects subsequently normalized, and four additional subjects had low total T3 levels suggesting a nonthyroidal mechanism. Seven subjects died during the first 4 months of follow-up compared with three in a control group (P < .001). Nine of the 40 subjects had a history of or current atrial fibrillation. No new atrial fibrillation was documented during 388 months of EKG follow-up. Low total T3 levels, TSH normalization, and excess mortality suggest that nonthyroidal illness plays a role in the pathogenesis of low TSH determinants in the nursing home. Autonomous production of thyroid hormone also plays a role. We believe that the term "subclinical hyperthyroidism" should be used only if the clinician believes that autonomous overproduction of thyroid hormone is the cause of a low TSH level. If subsequent research shows correctable adverse consequences associated with subclinical hyperthyroidism from autonomous overproduction of thyroid hormone, a more aggressive diagnostic approach will be needed to define the mechanism of a low TSH level at the time of its discovery.