In rats, taking the blood pressure drug propranolol seems to lower a key thyroid hormone, which might mean this type of medication affects how the body processes thyroid hormones.

If someone has an overactive thyroid, taking the heart medication propranolol might help lower certain thyroid hormone levels in their blood — possibly because the drug affects how the body converts one hormone into another.

Propranolol might lower more than just one type of thyroid hormone in rats — it could affect another key hormone too, meaning it has a wider effect on how the thyroid system works.

When rats are given the drug propranolol, their thyroid hormone levels go down — and this might be because the drug sticks to proteins in the blood, making the hormones less available. But we don’t have proof yet that this is exactly what’s happening.

If someone has an overactive thyroid, taking the heart medication propranolol might lower certain thyroid hormone levels, and how much it blocks the body’s ‘fight-or-flight’ system could be linked to how strong this effect is.

Even in people with healthy thyroids, taking the heart medication propranolol might change how their body processes thyroid hormones — raising some levels, lowering others.