While there are some studies on overfeeding that support the theory behind a well-controlled reverse diet, there is no research on reverse dieting and hypothyroidism. Therefore, we can only speculate as to how reverse dieting may impact those with an underactive thyroid.
Hypothyroidism may be caused by many things, the most common cause being Hashimoto’s thyroiditis, an autoimmune disease where your body attacks its own thyroid gland. An underactive thyroid may also be caused by things like iodine deficiency or subsisting on too few calories for too long.
It’s likely that the effectiveness of reverse dieting for those with hypothyroidism may partially be dependent on the cause.
For example, if you’ve been a chronic crash or yo-yo dieter and are subsisting on low calories, you may have a better chance of improving your thyroid hormone levels and increasing your metabolic rate than someone with Hashimoto’s. Studies have shown that thyroid hormones can plummet with aggressive dieting and increase when calories are restored for an extended period (such as what occurs during reverse dieting). But with Hashimoto’s thyroiditis, your low hormone levels and slow metabolism are caused by an attack that your body is waging on itself, so it’s unlikely that increasing your calorie intake will reverse this.
With no definitive data to draw from, if you have an underactive thyroid, it might be worth it for you to give a conservative reverse diet a try if your doctor approves.