Thydin Mechanism of Action

Pharmacology: Pharmacodynamics: Thyroid hormones influence most body systems as they are responsible for regulating growth and development, affecting metabolic processes, stimulating calorigenesis and having a positive chronotropic and inotropic effect on the heart. These systems may slow down when shortage of thyroid hormones occurs leading to fatigue, cold intolerance, sleepiness, dry skin, constipation and menstrual cycle disorder. In children, growth is delayed. Levothyroxine, the active ingredient in Levothyroxine Sodium Tablet (Thydin) is synthetically prepared sodium salt of the natural hormone thyroxine. The major component of normal thyroid gland secretion is Thyroxine (T4), but it has a very small pharmacologic effect itself. The amount of inactive T4 that is converted into the active triiodothyronine (T3) in the peripheral tissues is approximately 30%. The precise mechanisms by which the thyroid hormones exert their physiologic action are not well understood. However, the physiological effect mediating at the cellular level primarily by T3 is known.
Pharmacokinetics: Levothyroxine Sodium is variably but adequately absorbed from the gastrointestinal tract following oral administration. Fasting increases absorption. Only 0.03% of total thyroxine in plasma is free. Metabolism and excretion is reduced since thyroxine is bound to plasma proteins (Thyroxine Binding Globulin [TBG], Thyroxine-Binding-Pre-Albumin [TBPA] and albumin). Thyroxine has an elimination half-life of 6 to 7 days. In patients with hyperthyroidism, it is shortened to 3 to 4 days, whereas in patient with hypothyroidism and in elderly, half-life may be 9 to 10 days. In the liver, thyroid hormones are primarily degraded and their metabolites are excreted in the bile. Since thyroid hormones are released by hydrolysis in the intestine and reabsorbed, enterohepatic circulation occurs. Due to the long half-life of the T4, one single daily of Levothyroxine Sodium Tablet (Thydin) can result to a steady state blood level of the biologically active T3. When the oral dose is increased or decreased, the change in thyroxine levels is slow. The steady state level of thyroxine is reached in about a month.