When rats have an overactive thyroid, their hearts have more receptors that respond to stress hormones, which might make their hearts beat harder and faster when those hormones are present.

We analyzed the available evidence and found that 14 studies support the idea that rats with an overactive thyroid develop more beta-adrenergic receptors in their hearts. These receptors are the sites where stress hormones like adrenaline bind to trigger responses such as a faster or stronger heartbeat. The evidence we’ve reviewed suggests that when these receptors increase in number, the heart may respond more strongly to the same amount of stress hormone. This could mean the heart beats harder or faster than it would under normal thyroid conditions. No studies in our review contradicted this pattern. What we’ve found so far leans toward a connection between excess thyroid activity, increased receptor levels, and a heightened reaction to stress hormones in rat hearts. However, we cannot say this applies to humans or that the receptor increase directly causes every change in heart behavior — only that the pattern has been observed consistently in the studies we’ve examined. The evidence is limited to animal models and does not explain how or why the receptors increase, only that they do. For now, the clearest takeaway is this: in rats with hyperthyroidism, the heart appears to have more of these hormone sensors, and that may make it more reactive to stress signals.