Many conditions can lead to deficiency of thyroid hormones. It may be due to auto immune injury to gland or may be due to infections. In newborn and infants, it may be due to absence of thyroid gland . It is more common in females and family history is also important . In few patients , hypothyroidism is due to pituitary gland diseases .
Thyroid gland produces T4 and T3 hormones . Thyroid gland function is further regulated by pituitary gland through a hormone known as TSH. When T4 & T3 production goes down, it is sensed by pituitary and this gland now releases more TSH . Thus low T4, T3 and high TSH is hallmark of Hypothyroidism. While in less common pituitary origin hypothyroidism, T4,T3 and TSH all are low.