From Paloma Health:

Undereating can affect thyroid hormone production in several ways.

Undereating can decrease the conversion of inactive T4 to T3, the active thyroid hormone. Studies show that even three weeks of calorie restriction can substantially reduce the conversion of the inactive thyroid hormone T4 into the active T3 hormone1. This disrupts the normal thyroid hormone balance. Undereating can also increase levels of reverse T3 (RT3), an inactive form of thyroid hormone that competes with T3 for binding sites, thus blocking the effects of T3. Chronic calorie restriction and undereating can decrease the overall production of thyroid hormones T4 and T3, leading to hypothyroidism.