Always cold and gaining weight? - it could be 'invisible illness' often confused with 'getting old'

It's thought to affect more than one in 50 people yet with many sufferers failing to spot the symptoms doctors believe it could be much higher

In the freezing weather of the last few months many people have felt they could never get warm. And tucking in to comfort foods to cheer you up over the dark nights can quickly see your waistline expanding.

But while it is easy to put this down to the winter blues it might actually be caused by a completely different reason. For both gaining weight and constantly feeling cold can both be signs of an "invisible illness".

According to the NHS both are common signs of an underactive thyroid. But both, along with many of the other symptoms of the condition, are often mixed up with other health problems or even the menopause.

An underactive thyroid gland (hypothyroidism) is where your thyroid gland does not produce enough hormones. The most common signs of an underactive thyroid are tiredness, weight gain and feeling depressed.

Others include:

• being sensitive to cold
• constipation
• slow movements and thoughts
• muscle aches and weakness
• muscle cramps
• dry and scaly skin
• brittle hair and nails
• loss of libido (sex drive)
• pain, numbness and a tingling sensation in the hand and fingers (carpal tunnel syndrome)
• irregular periods or heavy periods

The NHS says: "Many symptoms of an underactive thyroid (hypothyroidism) are the same as those of other conditions, so it can easily be confused for something else. Symptoms usually develop slowly and you may not realise you have a medical problem for several years.

"For example, if you are an age where you may be expecting the menopause, you may think you have started the menopause rather than having an underactive thyroid."

An underactive thyroid is caused by the immune system attacking and damaging the thyroid gland. This makes it produce less of the hormone thyroxine. However some treatments for thyroid cancer of overactive thyroid can also cause it.

It can affect men although is is more common in women. Children can also develop an underactive thyroid and some babies are born with it.

Treatment for an underactive thyroid involves taking daily hormone replacement tablets, called levothyroxine, to raise thyroxine levels. Patients will need regular blood tests until the correct dose of the drug is reached. This can take a little while to get right.

If an underactive thyroid is not treated, it can lead to complications, including heart disease, goitre, pregnancy problems and a life-threatening condition called myxoedema coma (although this is very rare).

The only way to discover if you have an underactive thyroid is to have a thyroid function test, where a sample of blood is tested to measure your hormone levels, says the NHS.