It’s so much worse than just weight gain, although this is a legitimate symptom.
Your thyroid gland is located in the neck and is in charge of your metabolism and energy, and because of this, so many functions of the body rely on it to work properly. An underactive thyroid means just that; it’s under active. It’s slow and sluggish and not performing properly, meaning a slow metabolism that causes weight gain and very little energy, as well as lots of other symptoms. For the thyroid gland to function properly, we need the right amount of thyroid hormones in our bodies. If we don’t have these, it affects our energy levels and lots of other things, especially things you probably don’t even think about. For example, sleep cycles, body temperature, alertness, thinking clearly, appetite and fertility, to name just a few.
Hypothyroidism can be caused by an autoimmune disease, such as Hashimoto’s thyroiditis or Graves’ disease. Sometimes it is caused by having treatment for hyperthyroidism (an overactive thyroid) or having had treatment for thyroid cancer. The treatment often used for these, radioactive iodine treatment, can result in the patient becoming hypothyroid. So any of these could have caused the person you know to have the condition.