An underactive thyroid means your thyroid isn’t making enough thyroid hormone, which slows your whole metabolism down and leaves you tired, cold and gaining weight. It’s common, easy to mistake for something else, and it’s picked up with a simple blood test at your GP. The good news: for most people it’s very treatable. This is background reading, not a diagnosis, so if the symptoms sound familiar, ask your GP for a blood test.
What is an underactive thyroid?
An underactive thyroid, or hypothyroidism, is when your thyroid gland doesn’t produce enough thyroid hormone. That hormone runs more or less your entire metabolism: your energy, your body temperature, your weight and your concentration. When there’s too little of it, everything starts running on a lower setting.
By far the most common cause is an autoimmune condition called Hashimoto’s, where your own immune system slowly attacks and breaks down the thyroid. According to the endocrinologist in our podcast, this is behind more than 80 to 90 per cent of cases. Other causes are much rarer, such as previous thyroid surgery or radioactive iodine treatment for an overactive thyroid.
Symptoms to look out for
The best-known signs are tiredness and weight gain, but there’s a longer list. Commonly reported symptoms include:
- Fatigue and low energy
- Weight gain
- Feeling the cold easily
- Constipation
- Low mood
- Brain fog: struggling to think clearly and sharply
The tricky part is that these symptoms are very general and could be caused by a hundred other things. That’s exactly why a blood test is needed to know whether your thyroid is actually to blame.
How it’s diagnosed
On one level the diagnosis is straightforward, because there are blood markers that show clearly how your thyroid is doing. The key one is TSH, the signal your brain uses to tell your thyroid what to do. If your thyroid is running too slowly, your brain pushes harder and your TSH rises. With a clearly underactive thyroid, it isn’t 5 or 6 but 20, 30 or sometimes even above 100.
Doctors often also measure the thyroid hormone itself, T4 (and sometimes T3). In Hashimoto’s, TPO antibodies may show up in the blood too. One important distinction: having antibodies is not the same as having an underactive thyroid. The endocrinologist compares it to bullets: you can have the bullets, but they may not have been fired yet. Only once the thyroid is genuinely damaged and making less hormone does the condition itself develop.
Who is most at risk?
Women get an underactive thyroid far more often than men, roughly ten times as often. Other factors that raise the risk are getting older and the period around pregnancy and birth.
Iodine deficiency is a major cause worldwide, but in the UK, as in the Netherlands, it’s rarely the issue if you eat a reasonably varied diet. Most people get enough iodine, partly through dairy. It only becomes something to watch if you structurally avoid those foods and use only non-iodised salt.
The treatment: topping up the shortfall
The core of the treatment is simple: if you’re short of a hormone, you replace it. That’s done with levothyroxine, the synthetic form of thyroid hormone (T4). Your body converts that T4 into the active T3 itself. For the vast majority of people this works well, and they can get on with life without any real limitations.
One thing worth knowing: the hormone needs time to build up. You won’t wake up symptom-free the next morning. It takes roughly a month, sometimes a bit longer, before you’re back on track and start noticing a trend towards feeling better. Levothyroxine is best taken on an empty stomach.
Why it doesn’t work for about 1 in 5
In around 10 to 20 per cent of treated patients, symptoms stick around, mainly fatigue and low energy. On paper everything looks right: the TSH has dropped neatly back into the normal range. And yet the person still doesn’t feel well.
A big part of the explanation is the balance between T4 and T3. A healthy thyroid makes some T3 of its own. If you only top up with T4, you end up with relatively a lot of T4 and relatively little active T3 in your blood, a balance that’s slightly off from what your body used to do itself. On top of that, everyone is differently sensitive to thyroid hormone. The endocrinologist has seen patients with wildly abnormal readings and no symptoms, and others with only mildly abnormal readings who felt awful. Those individual differences make it hard to get everyone right on a single standard dose.
Combination treatment with T3
For people with lingering symptoms, a combination of T4 and T3 can be an option. This is emphatically not a standard treatment, but an individual decision you make together with your doctor.
The science is divided on it. Large studies show no clear average benefit, partly because they often looked at the whole group, while 80 to 90 per cent are already doing fine on T4 alone. At the same time, some studies suggest that when you ask people directly what they prefer, a majority favour the combination, and research indicates the combination is safe. In practice the endocrinologist sees a subset of people who genuinely benefit. He always starts with the lowest dose of T3 and builds up slowly until someone notices a difference. Because T3 acts quickly and causes peaks in the blood, it’s sometimes split across several moments in the day.
The honest caveat: it doesn’t help everyone. Sometimes there’s something other than the thyroid behind the symptoms.
The bottom line
An underactive thyroid is common, easy to overlook and, for most people, very manageable once it’s picked up. The route is always the same: if the symptoms sound like yours, don’t self-diagnose or lean on supplements. Ask your GP for a blood test. That’s the only way to find out what’s really going on.
Frequently asked questions
What are the symptoms of an underactive thyroid?
The most common symptoms are tiredness, weight gain, feeling the cold easily and constipation. Low mood and brain fog (struggling to think clearly) are also frequent. Because these symptoms are so general, a blood test at your GP is needed to confirm whether your thyroid is actually the cause.
Can you lose weight with an underactive thyroid?
An underactive thyroid slows your metabolism down, so you tend to gain weight more easily. Once levothyroxine brings your hormone levels back up, your metabolism often normalises and many symptoms improve. Don’t expect a miracle of a few kilos, though: thyroid hormone is not a weight-loss drug. Talk to your doctor about what’s realistic for your situation.
Why am I still tired despite taking thyroid medication?
In roughly 1 in 5 treated patients, symptoms like fatigue persist even when the TSH reading is perfectly normal. One explanation is the balance between T4 and T3 being slightly different from what your body made itself. Sometimes something other than the thyroid is involved. If you stay tired, raise it with your doctor; a combination treatment with T3 can be an individual option in some cases.
Can an underactive thyroid go away on its own?
With the most common cause, the autoimmune condition Hashimoto’s, the thyroid is gradually damaged and its function does not recover by itself. The endocrinologist is clear about this: the thyroid simply stops working properly and the hormone needs to be replaced. Treatment is therefore usually long-term or lifelong. Always have it assessed by your doctor through a blood test.
Do iodine and selenium supplements help with an underactive thyroid?
Iodine and selenium matter for making thyroid hormone, but a deficiency is rare on a reasonably nutritious diet in countries like the UK. Supplementing up to high levels has, in the endocrinologist’s view, questionable added value. The advice: don’t rely on supplements to sort your thyroid out. A genuine underactive thyroid usually isn’t fixed that way.
How is an underactive thyroid diagnosed?
Through a blood test at your GP. The key marker is TSH, which rises when your thyroid runs too slowly. Doctors often also measure the hormone T4, and TPO antibodies if Hashimoto’s is suspected. With a clearly underactive thyroid the TSH is markedly raised (for example 20 or 30 instead of around 5). If you have symptoms, ask your GP for a blood test.