Levo-T (levothyroxine sodium) has been approved by the FDA for the treatment of hypothyroidism and the suppression of thyroid stimulating hormone. Levo-T will be available in dosages ranging from 25 mcg to 300 mcg.
Synthetic levothyroxine (T4) is identical to that produced naturally in the human thyroid gland. Thyroid hormone synthesis and secretion is regulated by the hypothalamic-pituitarythyroid axis. Thyrotropin-releasing hormone (TRH) released from the hypothalamus stimulates secretion of thyroid-stimulating hormone, TSH, from the anterior pituitary. TSH, in turn, is the physiologic stimulus for the synthesis and secretion of thyroid hormones, L-thyroxine (T4) and L-triiodothyronine (T3), by the thyroid gland. Circulating serum T3 and T4 levels exert a feedback effect on both TRH and TSH secretion. When serum T3 and T4 levels increase, TRH and TSH secretion decrease. When thyroid hormone levels decrease, TRH and TSH secretion increase.
In July 2001, the FDA announced that it would require all manufacturers of levothyroxine sodium products to submit New Drug Applications for approval of these products. This guidline applies to both original and generic forms of the levothyroxine sodium drugs. As of March 2002, Levo-T was the only generic levothyroxine sodium product to be approved by the FDA.