The thyroid is a small gland located in our throat. This gland is responsible for making thyroid hormone. First , a hormone called TRH is made from the hypothalamus of the brain . TRH then stimulates another gland called the pituitary gland to produce a hormone called TSH from the pituitary gland. When this hormone stimulates the thyroid gland again , thyroid hormone is produced using iodine from food. There are two types of thyroid hormones - T3 ( 0.1%) and T4 ( 99.9%) , these two hormones play a role in performing many important functions of our body. If the hypothalamus , pituitary and thyroid gland have problems with any one of these three , the amount of thyroid hormone in the body will increase. Moreover, even if there is a lack of iodine , the amount of thyroid hormone will decrease. If the amount of thyroid hormone in the body decreases, it is called "hypothyroidism" and if it increases , it is called "hyperthyroidism". Weight gain is one of the common causes of thyroid problems. As well as this, the body weight can be reduced abnormally. The most common symptom of thyroid dysfunction is a change in body weight , which is not immediately explained. Excessive weight gain can lead to a decrease in the level of thyroid hormone , which is called 'hypothyroidism' in the language of medical science. On the other hand, if the thyroid continues to secrete excess hormones the body needs, then the body loses weight unexpectedly. This is called 'hyperthyroidism'.