Completely out of character, Jim Harwood found himself intensely stressed and angry about even the most trivial things.

Someone pushing a shopping cart in front of him at the grocery store or a minor error by a work colleague would trigger a harshly worded outburst which he would immediately regret.

At the same time, the razor-sharp focus and intense concentration he had always relied on as a computer programmer was starting to desert him. He struggled to keep his mind on the job, to remember names, to control his words and actions.

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The 73-year-old says: 'I just didn't know what was happening to me. I had a level of irritability – of stress and anxiety – that I just couldn't explain.'

As the symptoms got worse, he considered any number of possibilities – from the onset of dementia to even mad cow disease. One thing he never imagined, however, was that his problems were caused by his thyroid.

'It just wasn't something I suspected,' he says.

Yet experts have told the Daily Mail that psychological and cognitive disturbances are an under-recognized and frequently overlooked symptom of hypothyroidism – a condition that affects nearly five percent of Americans ages 12 and older.

And they have called on doctors to consider thyroid tests in patients struggling with symptoms such as depression, anxiety, memory problems and difficulty concentrating – to establish whether an underactive thyroid could be to blame.

Kristien Boelaert, professor of endocrinology at the University of Birmingham and a trustee of the British Thyroid Foundation, says: 'Hypothyroidism often goes undiagnosed. So if people go to their doctor with anxiety or depression – especially if they are showing any of the other more commonly recognized physical symptoms – then doctors should be prepared to suggest tests for thyroid function.'

So how can you know whether you have hypothyroidism? And how can it be treated?

First, it's important to explain what exactly the thyroid is. The thyroid gland is a small, butterfly-shaped organ found in the neck just in front of the voice box that produces crucial hormones needed to keep the body healthy.

In hypothyroidism, levels of these hormones are too low, which can lead to a wide range of physical symptoms including tiredness, weight gain, feeling cold, constipation, dry or thinning hair, muscle weakness, aches, a hoarse voice and slowed speech and movement.

However, because receptors for thyroid hormones are also found in the brain, research shows that hypothyroidism can even affect how people think and feel.

Lyn Mynott, chief executive of the nonprofit Thyroid UK, says: 'It's not as widely recognized, but if levels of thyroid hormones are low it can cause psychological issues. A lot of people can suffer these symptoms for a long time without realizing they're suffering from a thyroid disorder.'

Hypothyroidism is much more common in women and becomes increasingly common with age, particularly after 60.

Mynott says thyroid-related psychological issues are often misdiagnosed – especially in middle-aged women.

She adds: 'They might think the problems they're suffering are a product of their busy, stressful lives. If they're older, they might believe it's the start of dementia.

'But even when they do seek help, doctors often miss the real cause, as they don't think to do thyroid tests.

'As a result, patients might be prescribed an antidepressant or told to reduce their stress. But if the cause is actually hypothyroidism, these won't help or address the root of the problem.'

Experts say that, undiagnosed, hypothyroidism can cause serious health problems. Insufficient thyroid hormone can slow the heart rate and contribute to high cholesterol, increasing the risk of cardiovascular problems. For women, the condition can also disrupt ovulation, alter menstrual cycles and cause fertility problems.

In the most severe cases, long-term untreated hypothyroidism can cause myxedema coma – a rare but life-threatening condition in which the body's vital functions slow to dangerously low levels.

For Harwood, from Pinner in northwest London, doctors only realized his psychological problems were linked to his thyroid when he later developed a range of physical symptoms.

He says: 'I put on weight – an extra 20 pounds – and found myself becoming profoundly cold. Even in a heated office I needed to wear a thermal undershirt under a shirt and sweater.

'The fatigue was crippling. I could barely function.'

He also developed an enlarged thyroid, or goiter – another possible sign of thyroid disease.

Eventually he was prescribed synthetic thyroid hormone – a daily medication that has almost entirely eradicated his symptoms.

He says: 'It was only after the physical symptoms appeared that it became clear that hypothyroidism was the cause.

'Once I started on the medication, my mood completely lifted.

'It was really something. I just wish it hadn't taken so long.'

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