Even a little decrease in thyroid function may increase the risk of more severe heart failures, researchers, including one of Indian origin, have warned.
The new study found that patients with more severe heart failure have higher levels of the thyroid hormones TSH and T4 and lower T3 levels, and those with higher T4 levels may be more likely to have atrial fibrillation.
Researchers from University of Pennsylvania in the US investigated 1,382 patients with pre-existing moderate to advanced heart failure.
Study participants were on average 57 years of age of which 35 per cent were women. About 153 (11 per cent) were taking amiodarone and 174 (13 per cent) were taking levothyroxine.
Overall, fewer than one per cent of the patients were overtly hypothyroid, six per cent were subclinically hypothyroid, 88 per cent were euthyroid, five per cent were subclinically hyperthyroid, and one per cent were overtly hyperthyroid.
Researchers adjusted the statistical models for age, sex, race, body mass index (BMI), heart failure etiology, as well as amiodarone and levothyroxine (LT4) use.
They found that in patients with pre-existing heart failure, higher TSH, higher free T4 and lower T3 concentrations were each associated with more severe heart failure, while only higher free T4 was associated with atrial fibrillation.
Also in this population, subclinical hypothyroidism with TSH 7.0 milli-international units per liter (mIU/L) or higher was linked with worse survival, researchers said.
“Our results indicate that having subclinical hypothyroidism, a mild decrease in thyroid function, is associated with increased likelihood of needing mechanical assistance to the heart with devices, heart transplantation or death,” said lead author Lakshmi Kannan of University of Pennsylvania.
“We also found that blood tests commonly performed to assess thyroid function, including thyroid stimulating hormone (TSH) levels and two distinct thyroid hormones called thyroxine (T4) and triiodothyronine (T3), are associated with the severity of heart failure,” Kannan said.