Thyroxine hormone pill on its blister pack. This contains the L-isomer of the thyroid hormone thyroxine (T4), known as levothyroxine. The L form is metabolised more slowly than the D form, which increases its effect on the body. The drug is taken to treat thyroid hormone deficiency (hypothyroidism), which can cause retarded growth and development in children, and depression, fatigue and memory problems in adults.