Welcome to the forum, Sinha_Vikrant5.
Your sister is overtly hypothyroid and her GP should prescribe Levothyroxine as soon as possible. In the UK patients under 50 without heart disease can be prescribed 75-100mcg as a starting replacement dose. For patients with heart disease the starting dose will be 25mcg.
Thyroid replacement is titrated slowly so your sister should have a follow up thyroid test six-eight weeks after starting Levothyroxine as she may need dose increases until TSH is just above or below 1.0. She will almost certainly need to take thyroid replacement daily for the rest of her life.
For maximum absorption Levothyroxine should be taken with water 1 hour before or 2 hours after food and drink, 2 hours away from other medication and supplements, and 4 hours away from iron, calcium and vitamin D supplements and oestrogen.
Ask her GP to check thyroid peroxidase antibodies and thyroglobulin antibodies to see whether her hypothyroidism is caused by autoimmune thyroid disease (Hashimoto's). Cutting gluten from diet can be helpful in reducing Hashimoto's symptoms and antibodies.
I am not a medical professional and this information is not intended to be a substitute for medical guidance from your own doctor. Please check with your personal physician before applying any of these suggestions.