Underactive thyroid predicts dying from heart disease, not getting it

Whether an underactive thyroid harms the heart has been argued over for years, and the debate has often blurred two different questions. One is whether hypothyroidism makes cardiovascular disease more likely to develop in the first place. The other is whether, once heart disease exists, low thyroid function makes the outcome worse. Because thyroid hormone influences the cardiovascular system, both are plausible, but treating them as a single question may explain why past studies have disagreed. This review deliberately pulled the two apart.

The authors systematically searched PubMed/MEDLINE, Scopus and Web of Science for studies published between January 2014 and April 2026 that compared adults with overt or subclinical hypothyroidism against people with normal thyroid function. Nine studies covering 9,662 patients met the criteria. Separate pooled analyses were run for incident cardiovascular events, drawing on six studies with 5,795 participants, and for cardiovascular mortality, drawing on eight studies with 7,526 participants. Study quality was assessed with a recognised tool for prognosis research, and sensitivity analyses removed one study at a time to test whether the results held.

Hypothyroidism showed no significant association with developing cardiovascular events, with an odds ratio of 0.92. Cardiovascular mortality was another matter: hypothyroid patients had 2.73 times the odds of cardiovascular death, a result with little disagreement between studies that survived every leave-one-out check. The qualitative synthesis pointed the same way, with consistently worse outcomes in acute and high-risk settings including acute coronary syndromes, acute heart failure, spontaneous coronary artery dissection and coronary bypass surgery. The authors caution that the null result for events should not be read as evidence of no effect.

The split result helps reconcile a contradictory literature: studies asking who develops heart disease and studies asking who survives it were measuring different things. The pattern fits the hypothesis that thyroid dysfunction matters most after cardiovascular disease has begun, shaping prognosis rather than onset. Open questions remain, including whether treating hypothyroidism changes survival, whether overt and subclinical forms differ in their impact, and why the association concentrates in acute, high-risk situations. As pooled evidence from studies that followed people rather than testing treatments, it can show association but not cause.

For the many people who manage an underactive thyroid, this is not a reason to expect heart disease: the analysis found no increased odds of developing cardiovascular events. It is, however, a reason to see thyroid function as genuinely relevant to cardiovascular health, since low thyroid function was linked to markedly worse odds of surviving heart problems once they occur. That makes thyroid status a piece of information worth keeping visible in any conversation about heart risk, while remembering that an association is not the same as a cause.