An underactive thyroid gland is a condition known as hypothyroidism, in which your thyroid gland does not produce hormones properly.
About one in every 50 Britons is affected by hypothyroidism, according to Thyroid UK, and women are between five and 10 times more likely to be diagnosed than men.
Hypothyroidism is most often caused by an autoimmune problem, which means that the symptoms of an underactive thyroid gland can be very varied, affecting many different parts of the body.
The most well-known symptom, however, is unexplained weight gain and struggling to move any kilos.
Understandably, this can be a really frustrating experience as those with an underactive thyroid gland can try many different diets but still see no results.
People with hypothyroidism can diet and practice religious exercise, and yet see no weight loss, or even continue to take despite their efforts.
This can make people feel insecure about their weight and demotivated by the lack of results they see after going to the gym and avoiding comfort food.
If tests confirm you have hypothyroidism, your doctor may prescribe you hormone replacement tablets to raise your thyroxine levels.
It is important to talk to a doctor as soon as possible if you are concerned that you may be at risk for hypothyroidism because if left untreated it can lead to further complications including heart disease and pregnancy complications.