What is Liothyronine sodium (T3)?

Liothyronine Sodium (T3): Clinical Overview

Liothyronine sodium (T3) is a synthetic form of the thyroid hormone triiodothyronine that has a rapid onset of action, short half-life of approximately 2.5 days, and is almost completely absorbed (95% within 4 hours), making it useful for quick dosage adjustments but requiring more frequent administration than levothyroxine. 1

Pharmacology and Mechanism of Action

• Liothyronine (T3) is the active thyroid hormone that enhances oxygen consumption in most body tissues, increases basal metabolic rate, and affects metabolism of carbohydrates, lipids, and proteins 1
• Unlike levothyroxine (T4), liothyronine is not firmly bound to serum proteins, making it readily available to body tissues with maximum pharmacologic response occurring within 2-3 days 1
• T3 has a higher biological activity but shorter half-life compared to T4, which normally provides 80% of the daily T3 production rate through extrathyroidal conversion 2

Clinical Applications

• Liothyronine may be used in hypothyroidism treatment, particularly when patients continue to experience symptoms despite normalized TSH levels on levothyroxine alone 2, 3
• T3 measurement is particularly helpful in evaluating highly symptomatic patients with minimal FT4 elevations in thyrotoxicosis 4
• Liothyronine has been used as an alternative therapeutic option in patients treated with immune checkpoint inhibitors who develop hypothyroidism non-responsive to levothyroxine therapy 5

Administration and Dosing Considerations

• Due to its short half-life, traditional liothyronine formulations require twice-daily administration 2
• Serum T3 frequently rises to supranormal values in the absorption phase, which can be associated with palpitations 2
• Newer formulations like poly-zinc-liothyronine (PZL) have been developed to provide more stable serum T3 levels with a lower peak concentration and extended absorption profile 6

Monitoring and Safety

• When treating thyroid disorders with liothyronine, monitoring TSH and free T3 levels is essential 4
• Overdosage of liothyronine can cause severe thyrotoxicosis, as demonstrated in case reports of compounding errors resulting in 1000-fold dosing mistakes 7
• Patients with elevated T3 due to thyroiditis may transition to hypothyroidism within weeks to months, requiring close monitoring and potential treatment adjustment 4

Advantages and Disadvantages Compared to Levothyroxine

• Advantages of liothyronine:

  • Rapid onset of action (within hours) 1
  • Quick cutoff of activity allowing easier dosage adjustment 1
  • May improve symptoms in patients who remain symptomatic on levothyroxine alone 2, 3

• Disadvantages of liothyronine:

  • Shorter half-life requiring more frequent dosing 2
  • Risk of supranormal T3 peaks causing palpitations 2
  • Less clinical experience compared to levothyroxine 2

Clinical Pitfalls and Considerations

• Patients taking levothyroxine alone tend to have higher reverse T3 (rT3) levels compared to those on preparations containing liothyronine 3
• When treating hyperthyroidism, if there is uncertainty about whether primary or central hypothyroidism is present, hydrocortisone should be given before thyroid hormone is initiated 4
• Bioequivalence sometimes differs among generic and brand name thyroid hormone preparations, which may affect clinical response 2