An underactive thyroid means the thyroid gland, which is situated low on the front of the neck, is not producing enough thyroxine for the body, according to NHS Choices. This is also known as hypothyroidism. Thyroxine controls the body's metabolic rate.
Common symptoms experienced from an underactive thyroid include tiredness, weight gain, feeling cold and loss of libido, states NHS Choices. Hypothyroidism is usually the result of the body's immune system attacking the thyroid gland, mistaking it as harmful to the body.
Although there is no cure, the symptoms of hypothyroidism are easily controlled with medication called levothyroxine, which gives the body the required level of thyroxine it needs to function properly, reports.NHS Choices.