General Information

Synthroid has been approved as replacement or supplemental therapy for primary, secondary, tertiary and subclinical hypothyroidism

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.

Side Effects

Adverse reactions associated with levothyroxine therapy are usually symptoms of hyperthyroidism due to therapeutic overdosage.

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)

Additional Information

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.