Thyroid UK
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Heart rate of 45

I'm due a review of meds in a weeks time. I have only been taking meds for 7 weeks. This week I've been very tired and started to feel my heart beating more deeply. I started watching my heart rate and it has been between 45-52 most of the time. Should I be concerned about this and see doctor before review or wait to see what new blood results turn up.

7 Replies

You're a runner, so it's probably due to your fitness. When I was doing a lot of cycling, I got mine down to 38bpm. That was decades ago, and my heart rate has stayed in the low 40s, which is probably more to do with hypothyroidism -- although it was racing during a hyper swing, five years ago.

The elderly should be cautious of a slow heart rate. A friend was put on a beta-blocker, never monitored, and his very slow heart rate was spotted when he went to A&E about a minor accident. He ended up needing a pacemaker.

Is your tiredness perhaps due to over-training? Have you an idea of what your heart rate has been when feeling OK?


I have no idea what my heart rate was before thyroid treatment started but I know that I historically have low blood pressure. I know I've been training but my heart rate has went from a maximum of 189 beats to 132 during running.Im no athlete and still two stone over weight so can't imagine that it's because of that.


When we are hypothyroid our metabolism slows down thus we can have a slow heart rate - also athletes do as well, so as you didn't know your previous healthy heart rate you've nothing to compare. The fact that you are overweight also is a clinical symptom of hypothyroidism and I will give you a list and I'm sure you may have more.

Also, if hypothyroid, and as you exercise it depletes your T3, and it is T3 which is the only active thyroid hormone required in all of our receptor cells. T4 (levothyroixine) is inactive and should convert to sufficient T3 but may not do so if levo is not at an optimum for you, i.e. you feel well with no symptoms. That's why a Free T4 and Free T3 are so informative but rarely taken. I shall give you a link and you can see the reason:


Sounds like by trying to lose weight and get healthier I'm making my thyroid worst. I am extremely tired just now and feeling my heart beating is becoming uncomfortable. If I'm reading you correct if my meds aren't right I will feel symptoms of my under active thyroid more .


Yes you will be symptomatic for a while until GP gradually increases dose. I do hope your GP is generous with your doses and all vitamins/minerals are at an optimum. We are usually deficient in some if not all.

As your dose is gradually increased you should feel better - it may take a few months and weight can be lost but some members still have difficulties but a diet will probably be possible, whereas if very hypo weight cannot be lost and may still be gained until you reach an optimum.

We have to read, learn and ask questions as some doctors appear to refer only to the TSH and T4.

My heart was struggling before I was diagnosed and for a while afterwards.


Sounds like by trying to lose weight and get healthier I'm making my thyroid worst.

Yes, sadly you are. You are still in the early days of treatment. Although I couldn't see a Free T3 result in the ones you have posted, yours will be low.

Levothyroxine is T4. A healthy thyroid makes T4 but yours isn't healthy, you aren't making enough, and so you need it supplied in a pill.

However, T4 isn't the active hormone that the body needs, it is a prohormone. This means that it is the raw material for making the active hormone.

The active thyroid hormone is called T3. A healthy person makes it, as and when it is needed. Some of the body's T3 comes directly from the thyroid, but most of it is made by converting T4 in the tissues of the body that need it - in the liver or the gut for example.

Because your thyroid isn't healthy you can't make enough T4 and T3 for your needs. With sufficient T4 most people can make enough T3, but you still have low T4 levels.

T3 is needed by every cell in the human body. It is needed for bodily repair, keeping a cell's metabolism up and probably loads of other things I know nothing about.

One thing that uses up T3 is exercise. And I gather that you run. If you use up your limited supplies of T3 then you won't have enough for normal functions - breathing, keeping your heart beating, digesting your food, thinking...

Another problem is dieting. People need adequate food to be able to convert T4 into T3. When people are starved the body slows down conversion to protect itself and its resources, thus slowing down the body too.

Some people who are hypothyroid have found they lose weight more easily by keeping their calorie intake quite high. But seriously - give up dieting while your thyroid hormone levels are poor. And, until such time as you feel well, keep your exercise to fairly gentle things like walking, yoga, swimming fairly slowly. When you get your thyroid hormone levels up, then you can start (slowly) increasing your activity levels.


I started running then a month into the programme I was told I was under active. I haven't been dieting just changed my lifestyle to a healthy one and trying to be 100% gluten free. I'm really confused about what's right to do. I really don't want to give up running as I have started training towards 10K now and don't want to go back into my old lifestyle. I find it extremely hard to lose weight but I am losing it but sometimes at half a pound a week but it is going down. I have bloods next Friday but only for thyroid I am going to ask if they can check the vitamins to but I know my iron is low as I'm on meds for that. So complicated even when your trying to do the right thing.


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