L-Thyroxine is a synthetic form of the thyroid hormone thyroxine.1,2,3 In vivo, L-thyroxine (0.9 and 2.7 μg) inhibits synthesis and release of thyrotropin induced by thyrotropin-releasing hormone from the anterior pituitary in mice.1 It also reverses decreases in levels of circulating thymic serum factor (FTS) and the number of T rosette-forming cells in an old age-induced mouse model of hypothyroidism.2,3 Formulations containing L-thyroxine have been used in the treatment of hypothyroidism.
1.Bowers, C.Y., Schally, A.V., Reynolds, G.A., et al.Interactions of L-thyroxine or L-triiodothyronine and thyrotropin-releasing factor on the release and synthesis of thyrotropin from the anterior pituitary gland of miceEndocrinology81(4)741-747(1967) 2.Fabris, N., Muzzioli, M., and Mocchegiani, E.Recovery of age-dependent immunological deterioration in Balb/c mice by short-term treatment with L-thyroxineMech. Ageing Dev.18(4)327-338(1982) 3.Fabris, N., and Mocchegiani, E.Endocrine control of thymic serum factor production in young-adult and old miceCell Immunol.91(2)325-335(1985)