Home » Drug Information » FDA Approved Drugs » 2002
Medical Areas: Endocrinology
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Synthroid (levothyroxine sodium)
The following drug information is obtained from various newswires, published
medical journal articles, and medical conference presentations.
Company: Abbott Laboratories
Approval Status: Approved July 2002
Treatment Area: Hypothyroidism and suppression of thyroid-stimulating hormone
Synthroid has been approved as replacement or supplemental
therapy for primary, secondary, tertiary and subclinical
According to Abbott Laboratories, Synthroid is the number one
prescribed treatment for hypothyroidism and the second
most-prescribed medication overall in the United States. The formal
approval validates the safety and efficacy of the therapy, and it
allows physicians to continue prescribing it to patients.
Adverse reactions associated with levothyroxine therapy are
usually symptoms of hyperthyroidism due to therapeutic
Mechanism of Action
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. (From Synthroid prescription label)
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