When you're hypothyroid, your fatigue, difficulty losing weight, or continued weight gain can all be attributable to a drop in the metabolism that frequently accompanies a slowed-down thyroid. Even after your hypothyroidism is properly treated with thyroid hormone replacement, you may find that your metabolism has not bounced back to where it was before. This sluggishness in your metabolism can leave you feeling exhausted, and you may find you can't lose weight, despite a healthy low-calorie diet. But there are solutions!