Explain why an underactive thyroid could cause a person to have an increased body mass

Thyroid function and weight gain. The thyroid hormones are important for regulating weight. It is well-known than hypothyroidism (an underactive thyroid gland) can lead to slower metabolism, weight gain, and a higher body-mass index (BMI).

She dislikes her. She hates how her breath smells, she is calling her a wrinkled old bat. She doesn't like the smell of garlic in her breath

Explanation:

Not needed.

Edit:oops not one sentence but this should give you a idea still. You could combine it.