An underactive thyroid is one of those conditions a lot of people think they understand until they actually look into the full list of symptoms.
Most know it can cause tiredness or hair loss, but one lesser known symptom rarely gets mentioned: a slow heart rate. As Dr. Amir Khan shared on his No Appointment Necessary podcast, it’s worth understanding why it happens and when it becomes serious.
When the thyroid gland isn’t producing enough hormone, it doesn’t just affect energy levels. Thyroid hormone plays a role in regulating heart rate too, so an underactive thyroid can cause the heart to beat more slowly than it should. It’s rarely the symptom that sends people to the GP, but it can be one of the more serious ones sitting quietly underneath the more obvious signs.
Unfortunately, this symptom often gets missed.
An underactive thyroid slows down bodily functions generally, since the gland isn’t producing enough hormone to keep energy levels regulated properly. That’s why the more commonly reported symptoms tend to be tiredness, hair thinning, dry skin and constipation, all things people notice and often try to manage on their own before ever mentioning them to a doctor.
A slow heart rate rarely gets flagged in the same way, mostly because it isn’t something people notice day to day the way they notice fatigue or hair loss. Many people go undiagnosed for a while simply because the symptoms they do notice, exhaustion or hair falling out, feel more urgent to bring up than a heart rate they haven’t even registered as unusual.
It can become a medical emergency in rare cases.
In most cases, a slow heart rate linked to an underactive thyroid isn’t immediately dangerous, but it can become serious if thyroid hormone levels drop significantly. If that happens, the heart can slow down enough to cause a loss of consciousness, which is considered a medical emergency and needs urgent attention.
It’s also important to note that women are more likely to develop an underactive thyroid than men, largely down to differences in how hormones regulate the immune system between the sexes. That same immune regulation that makes women generally more effective at fighting off infections can also make it harder for the body to switch off certain processes when it needs to, including thyroid function.
Even people on treatment aren’t automatically in the clear.
Levothyroxine is the standard treatment for an underactive thyroid, a synthetic version of the T4 hormone that replaces what the body isn’t producing enough of naturally. It’s available on prescription only, and doctors typically start patients on a low dose before adjusting it based on regular blood tests.
Being on levothyroxine doesn’t mean a slow heart rate is off the table entirely, as Dr Khan points out. It can still happen if the dose is too low to properly manage the condition, if someone has only just started treatment and their body hasn’t adjusted yet, or if there’s a separate medical issue contributing to the slow heartbeat alongside the thyroid problem.
Why getting the dose right is trickier than it sounds.
A healthy thyroid adjusts its hormone output automatically throughout life, responding to changes like ageing, weight gain, pregnancy or increased exercise without a person ever noticing. Someone on a fixed dose of levothyroxine doesn’t have that same built in flexibility, so when their body’s needs change, the medication doesn’t adjust along with it.
That’s why regular blood tests matter so much for anyone on thyroid medication. Dosage requirements can change over time, and without monitoring, a patient could end up on a dose that’s no longer right for them without any obvious warning sign until symptoms like a slow heart rate start to appear.
What to watch for and when to get checked.
People managing an underactive thyroid are often encouraged not to dismiss the condition as something to simply take a daily tablet for and forget about. Getting the dose right is tricky, and it’s a balancing act between blood test results, symptoms, and lifestyle changes that can change over months or years.
Anyone experiencing ongoing tiredness, hair loss, dry skin or constipation alongside an underactive thyroid diagnosis should mention it at their next check up, even if the individual symptoms feel minor on their own. And if a slow heart rate is ever severe enough to cause fainting or loss of consciousness, that’s considered a medical emergency and needs immediate attention rather than waiting for a routine appointment.
