CONTEXT: Subclinical hypothyroidism is a common condition that may lead to impaired cardiac function.

OBJECTIVE: This study sought to examine the effects of levothyroxine treatment in patients with subclinical hypothyroidism and heart disease.

DESIGN: This was a register-based historical cohort study.

SETTING AND PARTICIPANTS: The study was composed of Danish primary care patients and hospital outpatients age 18 years and older with established heart disease who were diagnosed with subclinical hypothyroidism in 1997-2011. Patients were stratified according to whether they claimed a subsequent prescription of levothyroxine. Event rates and incidence rate ratios (IRR) were calculated by use of time-dependent multivariable Poisson regression models.

MAIN OUTCOME MEASURES: Measures included all-cause mortality and major adverse cardiac events (MACEs), defined as cardiovascular death, fatal or nonfatal myocardial infaction and stroke, and all-cause hospital admissions.

RESULTS: Of 61 611 patients with a diagnosis of cardiac disease having their first time thyroid function testing, 1192 patients with subclinical hypothyroidism (mean age 73.6 [SD ± 13.3] y, 63.8% female) were included, of whom 136 (11.4%) were treated with levothyroxine. During a median follow-up time of 5.6 y (interquartile range, 6.5 y), 694 (58.2%) patients died. Patients treated with levothyroxine displayed no significantly increased risk of all-cause mortality (adjusted IRR, 1.17; 95% confidence interval [CI], 0.90-1.52), MACE (adjusted IRR, 1.08; 95% CI, 0.80-1.45), or hospital admission (adjusted IRR, 0.94; 95% CI, 0.71-1.24), when compared with patients not treated with levothyroxine.

CONCLUSION: Levothyroxine treatment in patients with subclinical hypothyroidism and heart disease was not associated with a significant benefit nor risk of all-cause mortality, MACE, or hospital admission in this large real-world cohort study.