Hypothyroidism: when your thyroid is underactive

What hypothyroidism actually is

Your thyroid makes hormones that set the pace of your metabolism - the rate at which your body turns fuel into energy. In hypothyroidism it does not make enough of them, so that pace drops. Your heart beats a little slower, your gut moves a little slower, your thinking feels a little slower, and you feel every bit of it.

It is common. It becomes more common with age, and it affects women considerably more often than men. It is also, once diagnosed, one of the more treatable long-term conditions in medicine - which is genuinely good news and also, when you are eighteen months into feeling terrible, quite an irritating thing to be told.

Why it happens

In the UK the usual cause is autoimmune: Hashimoto's thyroiditis, where the immune system gradually damages the thyroid. This is why TPO antibodies are often checked when an underactive thyroid is found. It is not caused by anything you did.

The other common route is treatment for a previously overactive thyroid. Radioactive iodine and thyroidectomy both frequently leave the thyroid unable to produce enough hormone afterwards - an expected outcome rather than a complication, and one you should have been warned about beforehand. Some medicines, notably lithium and amiodarone, can cause it. Worldwide, iodine deficiency is the leading cause, though this is rare in the UK.

How it is diagnosed

With a blood test, read as a pair. TSH is the pituitary gland's instruction to the thyroid, and it rises when the thyroid is underperforming - the body shouting louder at an organ that is not responding. Free T4 is the hormone the thyroid actually produced, and it falls.

A high TSH with a low free T4 is the classic picture of overt hypothyroidism. A high TSH with a free T4 still inside its reference range is subclinical hypothyroidism: the signal has changed before the hormone has. Whether subclinical hypothyroidism is treated depends on how high the TSH is, whether you have symptoms, whether you have TPO antibodies, and whether you are pregnant or trying to conceive - which changes the calculation considerably.

One abnormal result is rarely enough to act on. See your results came back off for what usually happens next.

Treatment

The standard treatment is levothyroxine: a synthetic version of T4, the same hormone your thyroid makes, taken as a daily tablet. It replaces what is missing rather than stimulating anything, which is why it works so reliably and why it is generally taken for life.

Your starting dose depends on your weight, your age, how underactive your thyroid is, and whether you have heart disease. Older people and people with cardiac conditions are usually started low and increased slowly. NICE guidance is to recheck TSH about every three months until it has settled, then once a year - and it warns that TSH can take up to six months to come back into range if it was very high to begin with. Retesting earlier gives you a number that is still moving.

How and when you take it matters more than most people are told. Calcium, iron, and some indigestion remedies block its absorption, and coffee interferes with it too. Getting the most from levothyroxine covers the timing properly, and the medication timing tool will show you where the clashes are in your own routine.

When the numbers are normal and you still feel awful

This is the part that gets skipped, so let us be direct about it: it is a real and common experience, it is documented in the medical literature, and it is not you imagining things.

A meaningful minority of people on levothyroxine with a TSH squarely inside the reference range continue to have symptoms. Nobody fully understands why. Several things are worth genuinely investigating rather than accepting, and they are reasonable to raise:

• Where in the range you sit. The reference range is wide. Some people feel better in the lower part of it. This is worth discussing, though it is not a licence to push TSH below the range, which carries its own risks to the heart and bones.
• Something else running alongside. Low iron or ferritin, low vitamin D, low B12, coeliac disease and pernicious anaemia all cluster with autoimmune thyroid disease and all cause fatigue. They are ordinary blood tests.
• Absorption. If you take levothyroxine with coffee, or within a few hours of iron or calcium, you may not be getting the dose you think you are.
• Whether it is the thyroid at all. Depression, sleep apnoea, perimenopause and chronic fatigue can look very similar from the inside, and can coexist with a treated thyroid.

Liothyronine (T3) is sometimes raised at this point. It is prescribed for a small number of people in the UK under specialist supervision, but the evidence that adding it helps most people is genuinely mixed, and access varies by area. It is a conversation for an endocrinologist, not a decision to make on your own.

What you should not do is stop bringing it up. Describing symptoms so you're heard is about making that conversation land, and anxiety, palpitations and being believed is about what to do when it doesn't.

Living with it

Once your dose is right, most people feel substantially better and stay that way. You will have blood tests roughly annually, more often after a change or during pregnancy. You get free prescriptions in England once you have a medical exemption certificate for hypothyroidism - worth asking about, because nobody volunteers it.

Tell your clinician as soon as you know you are pregnant - the same week you get a positive test, not at the next routine review - and tell them too if you are planning to conceive. Levothyroxine requirements rise in pregnancy, often early and often substantially, and the dose usually needs increasing quickly.

When to seek help sooner

Severe untreated hypothyroidism is rare but serious. Call 999 or go to A&E if you or someone else becomes profoundly drowsy, confused, or unusually cold to the touch - particularly an older person with known thyroid disease during cold weather or after an illness. This can be a life-threatening emergency called myxoedema coma, and it needs treatment straight away rather than a routine appointment. See when to seek urgent help.