Hypothyroidism is a common endocrine disorder where in the thyroid does not make enough thyroid hormone.
Subclinical/mild hypothyroidism refers to the state of slightly increased serum TSH with normal serum FT4 in patients who are usually asymptomatic.
Most common cause of primary hypothyroidism is autoimmune thyroiditis or Hashimoto's disease.
Levothyroxine is the first-line agent for treatment of hypothyroidism.



  • 1st-line agent for treatment of hypothyroidism
  • May be used to manage myxedema coma
  • Safe for use in pregnancy
  • Actions:
    • Used as a replacement therapy in hypothyroidism to suppress TSH secretion
    • Long T½, slow onset, regular bioconversion which is appropriately regulated by the tissues to the biologically active compound T3
  • Effects: Safe & predictable effects
    • Requires several wk to achieve peak therapeutic effects w/ regular oral dosing
  • Due to different bioavailability of the various preparations of Levothyroxine, patients are encouraged to use the same brand of Levothyroxine
    • If a change in brand is needed, the patient’s serum TSH should be retested & dosage of Levothyroxine be re-titrated


  • Actions:
    • Short T½, rapid onset
    • Biologically active compound (no conversion is necessary)
    • Used as a replacement therapy in hypothyroidism to suppress TSH secretion
  • Generally not recommended for routine treatment of hypothyroidism due to its rapid onset & turnover which requires more frequent administration & increases the risk of hyperthyroidism
  • Useful in management of myxedema coma (when its rapid onset & short duration of action are required)
  • Effects: Require 3 days to achieve peak therapeutic effects
Note: Desiccated thyroid hormones (w/ variable proportion of T3 & T4), combinations of thyroid hormones & T3 alone are generally not recommended for treatment of hypothyroidism.
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