While lack of exercise never does a body any good, it can't be blamed as a cause of hypoT.
The leading cause of hypoT is Hashimoto's thyroiditis, an autoimmune disease that causes the thyroid to fail.
The next most common cause is treatment for Graves' disease or surgical removal due to thyroid cancer.
Less common or rare causes are: Some drugs, such as lithium or amiodarone; severe iodine deficiency (rare in industrialized nations); radiation treatments of cancer, acne, or thymus gland disorders; pituitary gland disorders; overconsumption of soy products or goitrogenic foods; neck trauma or whiplash; nuclear plant accidental exposure; toxic chemical exposure or other environmental toxic exposures.