My wife has underactive thyroid and also lack of hemoglobin. treatment has been continuing for 25 days but she suffers from hair loss.what's reason

Multiple causes: If she's only 21 years old and has anemia, the source of her anemia needs to be discovered and corrected. Most commonly in females, it it is due to heavy menstrual bleeding. Either way, anemia of any source can cause hair loss. Given her anemia, she should have her iron levels checked and possibly take iron supplements if needed. Similarly low thyroid hormones can cause hair loss.