Image copyright BURGER/PHANIE Image caption Tiredness and 'brain fog' are common symptoms of the condition

Hypothyroidism - or an underactive thyroid - affects one in 70 women and one in 1,000 men according to the NHS. But it can be a tricky disease to diagnose and treat. Dr Michael Mosley, of Trust Me I'm a Doctor, asks if sufferers are slipping through the net.

Someone emailed me the other day to ask me if I had ever considered the possibility that I might have hypothyroidism; an underactive thyroid. The reason he contacted me is because he had seen me on television and noticed that I have quite faint eyebrows, which can be a sign of this disorder.

I have none of the other symptoms such as weight gain, tiredness and feeling the cold easily, so I've decided not to go and get myself tested.

But if you do - and you think you could you have it - what should you do about it?

To get some answers I've been talking to Dr Anthony Toft, who is a former president of the British Thyroid Association.

He tells me that the thyroid gland is a bit like the accelerator pedal on your car. It produces hormones which help control the energy balance in your body. If it's underactive, then your metabolic rate will be slower than it should be. This means that you are likely to put on weight. Other symptoms can include feeling too cold or too hot, lacking in energy, being constipated, low mood, poor attention or "brain fog".

Image caption Dr Mosley's 'faint eyebrows' led one doctor to contact him about hypothyroidism

The main hormones involved are thyroid stimulating hormone (TSH), T4 and T3. TSH is released by the pituitary gland and tells your thyroid to get going.

In response your thyroid should release the hormones T4 and T3. T4 is converted in your body into T3, the active hormone that revs up your cells.

If you have symptoms of hypothyroidism then your GP will probably test your blood. The signs they're looking for are high levels of TSH, together with low levels of T4.

If your TSH is higher than normal this suggests that the gland that produces this hormone - the pituitary gland - is working hard to tell the thyroid gland to produce more hormone, but for some reason the thyroid gland is not listening.

The pituitary then ups its game and produces more and more TSH, but T4 levels stay low.

So if you have a high TSH coupled with a low T4, it's likely that the body is saying "I need more thyroid hormone!" but the thyroid gland isn't doing what it's being told. The result is hypothyroidism.

When this happens patients are often prescribed levothyroxine (T4). Symptoms diminish and patients are happy.

Image copyright Science Photo Library Image caption Scans can be carried out for more serious thyroid problems

So if it's so straightforward, why are there so many forums full of dissatisfied patients? Why do we at Trust Me get so many emails about this subject?

One of the issues with the blood tests is that there are no standard international reference ranges. In the UK, for example, we set the bar rather higher than many other countries. Certainly Dr Toft thinks that current UK guidelines are sometimes interpreted too rigidly.

"If the T4 is right down at the lower limit of normal," he says, "and the TSH is at the upper limit of normal, then that is suspicious. It doesn't often arouse suspicion in GPs, but it should."

He is also concerned that when a GP does diagnose an underactive thyroid, then patients are almost always prescribed a synthetic version of T4.

This works most of the time but in some cases the symptoms don't improve. This might be because with some patients the problem is not an underactive thyroid, but the fact that they can't convert enough T4 into the active hormone T3.

One way round this is to take T3 hormone in tablet form, but here price is a problem.

"The cost of T3 has escalated incredibly," says Dr Toft. "It's now about £300 for two months' supply of T3, whereas it costs pennies to make."

Find out more

Underactive thyroid (NHS Choices)

Trust Me, I'm A Doctor is on BBC Two at 20:00 GMT, Wednesday 8 February - catch up on BBC iPlayer

How can I tell if I have a thyroid problem and what can I do about it?

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The man with a titanium chest

So if you have been put on T4 and it doesn't work, what about asking for a trial of T3? Because it is so expensive your GP may well say no.

So instead some patients are going online and buying T3 from foreign websites. But it's important that if you are taking T3 you are being properly monitored, because it can cause serious side effects, including heart problems.

A slightly less expensive hormone supplement taken from the glands of cows and pigs is available. It contains both the T3 and T4 hormones, and there is a growing call to prescribe it for patients who don't respond to T4 alone. So does Dr Toft think patients should be offered this combination?

"I suspect that in time that's what will happen," he says. "The trouble is the evidence base is not as strong as we would wish it to be, and I suspect it will be a long time before we have sufficient evidence."

Dealing with thyroid problems can be complicated. If you've had a blood test and the results have come back normal, then you can ask to look at the actual numbers. But you may also have to accept that medication is not for you and lifestyle changes may be more appropriate.

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