What is the most common cause of hypothyroidism?
The most common cause of hypothyroidism is autoimmune hypothyroidism (Hashimoto thyroiditis). Autoimmune hypothyroidism is more common in women than men.
What causes Autoimmune Hypothyroidism?
The immune system in our body protects us from foreign agents such as bacteria, viruses, etc, by producing an immune response against them when they attack our body. Patients with autoimmune hypothyroidism, have minor genetic alterations that lead to the development of a confused immune system. When these patients get exposed to some antigens (immune triggers), their immune system produces a response that protects the body from this trigger but simultaneously also damages their thyroid gland. This damage leads to reduced production of thyroid hormones leading to hypothyroidism.
Let me explain autoimmunity with a simple example. If Ukrainian soldiers are guarding the line of control and a fight starts between Russian and Ukrainian soldiers, Ukrainian soldiers should kill only Russian soldiers. But if a Ukrainian soldier's vision is slightly compromised, while killing Russian soldiers, he may also kill another Ukrainian soldier in confusion. A similar phenomenon happens in autoimmune disorders including autoimmune hypothyroidism.
Note: Hypothyroidism most commonly occurs due to autoimmune thyroid damage.