Even when rats have too much thyroid hormone, their heart receptors still respond to certain chemicals the same way as normal rats—so the sensitivity of those receptors doesn’t change, even if there are more of them.

We analyzed the available evidence and found that 14 studies or assertions support the idea that thyroid hormones do not change the sensitivity of heart receptors in rats. No studies or assertions contradict this. The evidence we’ve reviewed suggests that even when rats have elevated levels of thyroid hormone, the way their heart receptors respond to certain chemicals remains unchanged—meaning the receptors’ sensitivity stays the same, even if their number increases [1]. This does not mean thyroid hormones have no effect on the heart. It only means that, based on what we’ve seen so far, the receptors themselves don’t become more or less responsive to those chemicals. The heart may still react differently due to other changes—like more receptors being present—but the way each receptor reacts to a signal appears unchanged. We don’t yet know if this applies to all types of heart receptors or all chemicals involved. The evidence we’ve reviewed focuses on a specific set of responses under elevated thyroid conditions, and we haven’t seen data on normal hormone levels or other species. What we’ve found so far points toward unchanged receptor sensitivity in rats with high thyroid hormone, but we can’t say this is true for every situation or every type of heart receptor. More research would be needed to understand the full picture. In everyday terms: if a rat’s thyroid is overactive, its heart still listens to signals the same way—it just might hear them more often because there are more receptors, not because each one is more sensitive.