The key difference between levothyroxine and liothyronine is that levothyroxine is a synthetic thyroxine (T4) used to treat hypothyroidism and has a longer duration of action, while liothyronine is a synthetic triiodothyronine (T3) used to treat hypothyroidism as an immediate supplementation.
Hyperthyroidism is a disease characterized by decreased levels of thyroxin (T4) and triiodothyronine (T3) with increased levels of thyroid-stimulating hormone (TSH). Levothyroxine and liothyronine are used as synthetic thyroxine (T4) and triiodothyronine (T3), respectively, to treat hyperthyroidism. During the treatment, levothyroxine replenishes the thyroxine pool, while liothyronine increases the conversion of thyroxine to triiodothyronine. Consequently, T3 and T4 levels will be increased, resulting in the upregulation of DNA transcription and protein synthesis in the body.
CONTENTS
1. Overview and Key Difference
2. What is Levothyroxine
3. What is Liothyronine
4. Similarities – Levothyroxine and Liothyronine
5. Levothyroxine vs Liothyronine in Tabular Form
6. Summary – Levothyroxine vs Liothyronine
What is Levothyroxine?
Levothyroxine is used to treat all three forms of hypothyroidism, namely primary, secondary, and tertiary hypothyroidism. It is the synthetic form of naturally occurring thyroxine. Once the drug is administered, it promotes the conversion of thyroxine into L-triiodothyronine, thereby restoring the functions of thyroid hormones, such as glucogenesis, protein synthesis, and mobilization of glycogen storage in the body.
Figure 01: Chemical Structure of Levothyroxine
Multiple dosage forms, including oral tablets and capsules, solutions, and parenteral, are available with levothyroxine. Once administered orally, the drug is primarily absorbed from the jejunum and upper ileum. The absorbed drug displays a high affinity with plasma proteins such as thyroxine-binding globulin (TBG), albumin (TBA), and thyroxine-binding prealbumin (TBPA), which results in slower metabolic clearance and longer half-life. Levothyroxine is primarily metabolized in the liver and excreted through renal clearance.
What is Liothyronine?
Liothyronine is another synthetic thyroid hormone used to provide triiodothyronine (T3) directly, bypassing the need for the conversion from the precursor T4 hormone. Therefore, the drug is prescribed where a rapid therapeutic response is required as an adjunct therapy to surgery and as a diagnostic intervention in suppression tests for mild hyperthyroidism. Once the drug is administered, liothyronine replaces the endogenous thyroid hormone and binds with the thyroid receptor to control DNA transcription and protein synthesis. It also increases the metabolic rate, enhances energy production, and promotes the growth and development of tissues.
Figure 02: Chemical Structure of Liothyronine
Liothyronine is well absorbed orally, and the absorption rate does not depend on the concomitant administration of foods. The maximum plasma concentration is achieved at about 2.5 hours after administering liothyronine, and the half-life is reported to be between 1 and 2 days. The drug displays a high binding affinity with plasma proteins such as thyroxine-binding globulin, thyroxine-binding prealbumin, and albumin. Liothyronine is primarily metabolized in the liver, where it is deiodinated to monoiodothyronine and diiodothyronine and subsequently conjugates with glucuronides and sulfates. It is mainly eliminated via the kidney, in which less than 2.5% is excreted as an unchanged drug.
What are the Similarities Between Levothyroxine and Liothyronine?
- Levothyroxine and liothyronine are used to treat hypothyroidism as hormone replacement therapies.
- They are naturally produced in the thyroid gland.
- Both drugs are considered gluten-free.
- They are primarily administered orally in tablet form.
- Both drugs shared common side effects, such as minor hair loss for a short time, palpitation, increased heart rate, nervousness, and change in appetite.
What is the Difference Between Levothyroxine and Liothyronine?
There is a distinct difference between levothyroxine and liothyronine, even though they are synthetic thyroid hormone medications used for treating hypothyroidism. Levothyroxine is a stable standard treatment converted to active T3 in the body. In contrast, liothyronine is used for rapid response or impaired conversion, directly providing active T3. Moreover, levothyroxine has a longer half-life and consistent hormone levels, allowing once-daily dosing, whereas liothyronine has a shorter half-life associated with hormone level fluctuations and requires multiple daily doses. Food effect and dosage flexibility also vary between the two medications.
Below is a summary of the difference between levothyroxine and liothyronine in tabular form for side by side comparison.
Summary – Levothyroxine vs Liothyronine
Levothyroxine and liothyronine are synthetic thyroid hormone medications used for treating hypothyroidism. Levothyroxine is a stable standard treatment converted to active T3 in the body, has a longer half-life, once-daily dosing, and consistent hormone levels. Liothyronine is used for cases requiring a rapid response or impaired T4 to T3 conversion, directly providing active T3 with a shorter half-life and requiring multiple daily doses. They differ in food effect, treatment approach, duration of action, stability, dosage flexibility, conversion in the body, and side effect profile. Therefore, healthcare professionals should carefully consider the difference between levothyroxine and liothyronine when prescribing and managing hypothyroidism.
Reference:
1. Escobar-Morreale, Héctor F., et al. “Treatment of Hypothyroidism with Combinations of Levothyroxine plus Liothyronine.” The Journal of Clinical Endocrinology & Metabolism, vol. 90, no. 8, 2005, pp. 4946–4954.
2. Jonklaas, Jacqueline, et al. “Evidence-Based Use of Levothyroxine/Liothyronine Combinations in Treating Hypothyroidism: A Consensus Document.” European Thyroid Journal, vol. 10, no. 1, 2021, pp. 10–38.
Image Courtesy:
1. “Liothyronine” By Boghog – Own work (Public Domain) via Commons Wikimedia
2. “Levothyroxine 200” By Emeldir (talk) – Own work (Public Domain) via Commons Wikimedia