What is the half-life of thyroid hormones, specifically triiodothyronine (T3) and thyroxine (T4)?

From the Guidelines

The half-life of thyroid hormones differs significantly between triiodothyronine (T3) and thyroxine (T4), with T3 having a relatively short half-life of approximately 1-2 days, while T4 has a much longer half-life of about 7-10 days. This difference in half-life explains why T4 (levothyroxine) is typically the preferred medication for thyroid hormone replacement therapy, as it provides more stable blood levels with once-daily dosing. The longer half-life of T4 occurs because it binds more strongly to carrier proteins in the blood, particularly thyroxine-binding globulin (TBG), which protects it from rapid elimination. T3, being less protein-bound, is cleared more quickly from circulation. In the body, T4 functions primarily as a prohormone that is converted to the more metabolically active T3 in peripheral tissues through enzymatic deiodination. This physiological conversion process helps maintain consistent thyroid hormone activity despite fluctuations in hormone levels, ensuring stable regulation of metabolism, growth, development, and other important bodily functions.

Some key points to consider:

• T3 has a short half-life, which means it is quickly cleared from the body 1.
• T4 has a longer half-life, making it a more stable choice for replacement therapy 1.
• The binding of T4 to thyroxine-binding globulin (TBG) protects it from rapid elimination 1.
• T4 is converted to T3 in peripheral tissues, helping to maintain consistent thyroid hormone activity 1.
• The difference in half-life between T3 and T4 is important for understanding the regulation of metabolism, growth, and development 1.

Overall, understanding the half-life of thyroid hormones is crucial for effective management of thyroid disorders and maintaining optimal thyroid function.

From the FDA Drug Label

The biological half-life is about 2-1/2 days. The higher affinity of levothyroxine (T4) for both thyroid-binding globulin and thyroid-binding prealbumin as compared to triiodothyronine (T3) partially explains the higher serum levels and longer half-life of the former hormone Table 10. Pharmacokinetic Parameters of Thyroid Hormones in Euthyroid Patients Hormone Ratio in Thyroglobulin Biologic Potency t 1/2(days) Protein Binding (%) * Levothyroxine (T4) 10 to 20 1 6 to 7 ** 99.96 Liothyronine (T3) 1 4 ≤ 2 99. 5

The half-life of thyroid hormones is as follows:

• T3 (Liothyronine): 2-1/2 days or ≤ 2 days
• T4 (Levothyroxine): 6 to 7 days 2, 3

From the Research

Half-Life of Thyroid Hormones

The half-life of thyroid hormones, specifically triiodothyronine (T3) and thyroxine (T4), can be understood through various studies.

• The half-life of T3 and T4 is not directly stated in the provided studies, but we can infer some information about their kinetics.
• A study on the kinetics of thyroid hormones in acute liver insufficiency found that the plasma half-life of T3 and T4 was altered in sheep with induced hepatic insufficiency 4.
• Another study on myxoedema coma found that plasma T4 and T3 reached a peak upper normal range within 3 hours after administration of intravenous L-thyroxine, and then diminished slowly over 5-9 days 5.
• However, the exact half-life of T3 and T4 is not provided in these studies.

Kinetics of T3 and T4

• The kinetics of T3 and T4 were studied in ewes, and the data was adjusted to the open two-compartment model 4.
• The study found that the hepatic insufficiency provoked a decrease in the disposition consent of T3, an increase in the plasma half-life of the distribution phase, as well as a moderately decreased turnover rate of this hormone.
• The T4 kinetics were altered during acute hepatic insufficiency, with a decrease in the elimination constant K13 and hormone plasma turnover rate.

Clinical Implications

• The production of thyroid hormones, both thyroidal and peripheral, has clinical implications 6.
• Extrathyroidal T4 conversion to T3 is decreased in patients with different acute and chronic illnesses.
• When T4 conversion to T3 is impaired and serum T3 concentrations decline, serum concentrations of biologically inactive 3',5'-triiodothyronine (reverse T3) increase.
• The value and interpretation of various iodothyronine measurements are important in understanding thyroid hormone production and kinetics 6.