Could a young adult have dementia caused from untreated hypothyroidism?

Yes: Hypothyroidism is one of the most important metabolic causes of reversible cognitive impairment. The term refers to thyroid under function within adults, which results in deficits of the thyroid hormones, thyroxine T4 and triodothyronine t3 (liothyronine). This underfunction may originate in the thyroid itself or in the pituitary or hypothalamus which controls the thyroid gland.