Do you have an underactive thyroid?

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".

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.

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. "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.