Underactive thyroid (Hypothyroidism) - Causes 

Causes of underactive thyroid 

An underactive thyroid (hypothyroidism) is when your thyroid gland doesn't produce enough of the hormone thyroxine, also called T4.

Most cases of underactive thyroid are due to either the immune system attacking the thyroid gland or a damaged thyroid.

Immune system

Most cases of underactive thyroid happen when the immune system, which normally fights infection, attacks the thyroid gland. This damages the thyroid, which means it is not able to make enough of the hormone thyroxine, and leads to the symptoms of an underactive thyroid.

A condition called Hashimoto's disease is the most common type of autoimmune reaction that causes an underactive thyroid.

It is not clear what causes Hashimoto’s disease, but the condition runs in families. It is also common in people with another disorder related to the immune system, such as type 1 diabetes and vitiligo.

Previous thyroid treatment

An underactive thyroid can also occur as a side effect or complication of previous treatment to the thyroid gland, such as surgery or a treatment called radioactive iodine therapy.

These treatments are sometimes used for overactive thyroid (where the thyroid gland produces too much hormone) or thyroid cancer.

Less common causes

Worldwide, a lack of dietary iodine is a common cause of an underactive thyroid because your body needs iodine to make thyroxine. However, iodine deficiency is uncommon in the UK.

In some cases, babies are born with an underactive thyroid because the gland does not develop properly in the womb. This is called congenital hypothyroidism and it is usually picked up during routine screening soon after birth. 

A problem with the pituitary gland could lead to an underactive thyroid. The pituitary gland is located at the base of the brain and regulates the thyroid. Therefore, damage to the pituitary may lead to an underactive thyroid.

An underactive thyroid has also been linked to some viral infections or some medications used to treat other conditions, such as:

  • lithium - a medication sometimes used to treat certain mental health conditions, including depression and bipolar disorder
  • amiodarone - a medication sometimes used to treat irregular heartbeats (arrhythmias)
  • interferons - a class of medication sometimes used to treat certain types of cancer and hepatitis C

Speak to your GP or specialist if you are concerned that a medication you are taking may be affecting your thyroid hormone levels.

Page last reviewed: 02/07/2013

Next review due: 02/07/2015


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The 10 comments posted are personal views. Any information they give has not been checked and may not be accurate.

Kate1970 said on 09 October 2014

Ive been taking Levothyroxine daily for about the last 7 years and went to pick up my repeat prescription a few days ago - 6th October (which I was actually late in doing as Ive got a brain like a sieve!) and apparently all of a sudden someone had decided that the dosage should be reduced.
My last blood test was actually in June, I have already had one complete lot of prescription for my normal dosage since then so I couldn't understand what had happened! I spoke to the doctors receptionist and she said the doctor who did the repeat prescription had decided I was on too much. I asked, when had he decided that? and she said Sep 26th...but I haven't been in since my blood test in June! so how come? (in the meantime I was feeling rubbish, really emotional and vague - a bit like I'm behind a net curtain all the time and that was only with a gap of 3 days by this point) I was really starting to panic and thought my business is going to go down the pan if I have to feel like this from now on...
Anyway, doctor rang me last night and actually he was unbelievably marvellous - he said that the only thing the test is 100% good for is to see if I am actually taking the tablets at all!!
He said as far as he's concerned its how I feel that's important, not so much the results (within reason), so the good news is that as of this morning Ive got my full dose again and feel miles better already. But I bet some people might have just taken the pharmacist's word for it and not pressed the issue but Im so relieved that I did.

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ChristineHB said on 09 September 2013

I have hypothyroidism and hypoparathyroidism due to surgery. I take thyroxine. Recently I had to come of HRT and now I am feeling really tired. Does anyone know whether HRT withdrawal effects the levels of thyroxine?

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tara dionne said on 16 May 2013

try using bladder wrack look it up on the internet it's a type of seaweed also known as iodine,kelp or even (fucus visiculosus) I went to see a herbalist that prescribed this for me just like the same as nhs medicine would apply and my thyroid has been perfect ever since and I was diagnosed with underactive thyroid 4 years ago after the birth of my first daughter.I have since had another two children so both times again it went under active because they advise you not to take take during pregnancy! but it's completely safe to use if you breastfeed I know this because I was breastfeeding when the doctor/herbalist prescribed this treatment.she also gave me hema plex which is a food supplement for total blood health(Amazing is the only word I can find to describe the amont of energy I have my hair is lovely and thick before I was balding at the front of head before,lost loads of weight,never tired and I also look after 3kids all under age of 5! my nails are nice and long no longer brittle and most importantly the doctor done the T4 checks on my underactive thyroid and it was normal.I'm not saying that this treatment will work for everyone but what's the worst that can happen.if you don't like it go back onto your pills you were taking before.I have an underactive thyroid and have done for 4 years now and I barely notice I've got it.this is my first time on this site just happened to click on this site by mistake and then typed in underactive thyroid and read some of your comments I was just about to come off the site and thought to myself don't be so selfish you have a solution to the problem that might help a few other people suffering on this site so help them(please let me know if I'm aloud to forward on the name of the lady who prescribes me the tincture and hema plex because I'm not sure if your aloud to pass herbalists information over this site?hope I've helped a few of you out

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Maddyann said on 26 September 2012

I am 46 and have suffered with UT since 1989 after my second son was born. My condition has gotten worse over the years and my medication has gone from 50mg to 300mg of thyroxine. So I advise anyone who is suffering with their UT to either change doctors or tell there doctor how they want treating as when I first went to my doctors the first doctor refused to test me even though I have a family history as my mother, Grandmother and Uncles all suffer with this condition. I saw a different doctor and he was brilliant and made sure i'm tested regulary.

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tarcoco said on 06 August 2012

I am femail in my 60's and moved home from England to W. Wales and my problems began - I have had an underactive thyroid for 25 years and was on Thyroxin before I moved. My new doctor took a blood test when I registered with him (this was just 3 weeks after having a blood test done with my old doctor to make sure all was well before I moved) and has insisted that my thyroid is overactive and has cut my medication dosage - I also suffer with depression caused by a chemical imbalance and the effect that this doctor has made to my life is terrible. I have lost most of my hair and eyebrows, my skin is so dry and itchy that I am almost red raw in places where I have scratched (he says its dermatitus - which just means skin irritation!!) my dear husband I know is going through hell with my moods and all I want to do is cry and sleep and boy do I sleep!! I have joined a gym to try to improve my depression and to try to lose some of the weight I have piled on. I just didn't know what else to do till I read the other comments and now know that I can change my doctor and maybe get one that listens to me. My previous doctor always said that he listens to his patience and it's more of how they feel on their dose of thyroxine rather than test results within reason. This is my next task to find a doctor that listens before I just give up!! I feel just like sheba12 I am at the end of my tether and honestly just don't know how much more I can take!

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SusanF7758 said on 04 June 2012

To DITHW Thank you.... I realised on reading your comment that I too have been guilty of doing the same thing to my partner.
The vagueness that comes over when my thyroid starts playing up again does put a strain on the relationship. My partner gets frustrated and try's to control me because I can't seem to organise myself and forget what I am supposed to be doing, which in turns causes me frustration being so independent and used to being organised, I feel so lost not being able to concentrate and focus...its just not me, where have I gone!!
This Thyroid problem is a pain! It took 2 years to get the Doctor to believe in me, that I wasn't a hypochondriac, in the end I went and told him what I wanted testing after doing the research myself but in the meantime it got worse before things were done. So now the brain slows and I forget things but at last I have a woman Doctor who understands and listens. So now the medication has been doubled to 100mg daily and at last my head is clearing and although I am still writing everything down at least I can think!
So to all you out there who are suffering the same...
Dry skin
Weight gain
Etc etc etc your not alone and either change your Doctor or demand to try increasing your dosage its your body and only you can know how you feel.
Tests phuh mine was borderline and until the new Doctor put me back on the tablets I felt terrible. Now 2 months later I am on my way back. So good luck everyone and don't except, demand your rights for testing and trying.

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DITHW said on 30 April 2012

This may be unusual however I feel that it is worth knowing how the male partner of the sufferer feels. My partner is another victim of apathy from her GP and has suffered from UT for a number of years in my opinion. What I wanted to say that it is very difficult living with someone who suffers with this and despite all that I try to do she does not see it. Now I acknowledge that we men in general and not good at dealing with emotional stuff but I have tried so hard .Her moods, temper, depression and apparent amnesia has once again raised its ugly head and I am being blamed for everything whilst she is unable to acknowledge her behaviour. Lately having visited this site I am comforted hugely after reading of the side effects of UT nearly all of which my partner suffers with. I feel helpless at the moment but thank fully she has found a consultant and Osteopath who agree she should be on Thyroxine but this is a few weeks away before we next meet the specialist.
I love her very much (or should I say the gentle, soft and unaffected side) but this has changed me and I feel depressed and de-energised and sit here typing this wanting to pack my bags but she needs me and hopefully we can get through it. So ladies if you can please at least consider what your partner is having to deal with which is not as bad as having the condition can be just as destructive. I have even considered if I am suffering with an UT- god help me.

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trixxyv5 said on 25 March 2012

I'm a 26 year old woman, and after having swine flu in 2009 my health has got quite bad. I had a routine blood test after I started getting sciatica in my back in october 2011 which showed I was slightly anemic and was borderline underactive thyroid. I had another blood test 2 months later and my doctor said my thyroid had got slightly worse but I was still only borderline so wouldn't put me on meds. She said to have another blood test in 3 years but surely if it got worse in 2 months it could get a lot lot worse in 3 years? Also, I've become a lot more tired lately, I need to sleep in the afternoon even though I sleep well at night. My hair is also falling out at the root more, my skin is ver dry and flaky and I feel I have no energy.I have also gone off sex and just seem irritable all the time.I did suffer depression about a year ago too. I did have a minor sinus operation over 2 weeks ago so I'm not sure if that's why I feel like I do. I've been back and forth to the docs for various things lately and don't want them thinking I am a hypochondriac but am getting slightly worried as I've heard what it can do if left untreated.

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Tulip7 said on 08 February 2012

This is the first time I have ever posted a comment on a website, but after reading your comment I thought I would register, so I could respond. I am a woman of 42 (43 in April) and have been on thyroxine since the age of 3. I should have been on thyroxine since birth, but the doctors would not believe my Mum when she said there was something that wasn't quite right. For a long time, on and off, my thyroxine levels have been up and down and I have been alternating between dosages between 150 and 175 daily. I understand how hard it can be to get the balance right. For example, 5 years ago, my dose was raised upto 200mg daily and then after a while I suffered with very bad heart palpitations for a week, as I obviously did not need so much thyroxine in my system at the time. From past experience, I understand what you mean about some individuals in the medical profession being unsympathetic/not understanding. I had been thinking about alternating different sizes of pill, cutting smaller ones up for instance. Last year I changed my Doctor's surgery and a Doctor at the new surgery suggested I try taking 175mg on weekdays and 150mg at the weekend. I had a blood test in January and had a 'Good T4 result'. Is it possible for you to change your Doctors's surgery or get a second opinion or fresh look at your situation? I sincerely hope you mange to get it sorted out. Best wishes.

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sheba12 said on 17 January 2012

Im a woman of 45years old & have had hypothyroidism since I was only 3 months old,I hate it,sometimes the medical profession are so unsympathetic,il tell you why,over the last seven years ,my condition has got worse,I had to give up working because I was sleeping at least 17 hours a day,was physically exhausted& drained and so depressed because of it,I was on a certain dose and I was getting a bit better ,had bit more energy&wasnt sleeping so much and was happier,I went for a blood test and to my horror my gp said my levels were too high ,so they reduced the dosage and back to my old ways,im so depressed coz of it and dont know how much more I can take!!!!Help sheba12

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Overactive thyroid

Overactive thyroid, also known as hyperthyroidism, is a relatively common condition that occurs when there is too much thyroid hormone in the body