What is the significance of Triiodothyronine (T3)?

From the Guidelines

T3 should not be used as first-line therapy for hypothyroidism, but rather as an adjunct to levothyroxine (T4) in patients who do not respond well to T4 alone, due to its shorter half-life and potential for significant side effects. When discussing T3, it's essential to consider its role in regulating metabolism, body temperature, and heart rate. T3 is available as liothyronine sodium (Cytomel) in doses typically ranging from 5-50 mcg daily. However, levothyroxine (T4) is preferred as first-line therapy because it converts naturally to T3 in the body and provides more stable hormone levels, as noted in studies on thyroid disease screening 1.

Some key points to consider when evaluating T3 include:

• T3 has a shorter half-life (about 1 day) compared to T4 (about 7 days), requiring more frequent dosing
• Side effects of excessive T3 include heart palpitations, anxiety, insomnia, weight loss, and heat intolerance
• T3 levels should be monitored through blood tests, and dosage adjustments should only be made under medical supervision, as thyroid hormone imbalances can have significant health consequences
• Patients at higher risk for thyroid dysfunction, such as the elderly, postpartum women, those with high levels of radiation exposure, and patients with Down syndrome, should be closely monitored for subtle signs of thyroid dysfunction 1.

In terms of specific patient populations, subclinical hypothyroidism is associated with poor obstetric outcomes and poor cognitive development in children, highlighting the importance of careful monitoring and management of thyroid hormone levels in these patients 1. Additionally, subclinical hyperthyroidism has been associated with atrial fibrillation, dementia, and osteoporosis, emphasizing the need for careful evaluation and treatment of thyroid dysfunction in patients with these conditions 1.

From the FDA Drug Label

The mechanisms by which thyroid hormones exert their physiologic action are not well understood. These hormones enhance oxygen consumption by most tissues of the body, increase the basal metabolic rate and the metabolism of carbohydrates, lipids and proteins Thus, they exert a profound influence on every organ system in the body and are of particular importance in the development of the central nervous system. T3 is almost totally absorbed, 95 percent in 4 hours. 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 T3 contains three atoms of iodine and is formed by the coupling of one molecule of DIT with one molecule of monoiodotyrosine (MIT).

T3 (Liothyronine) is a synthetic form of a natural thyroid hormone. It is almost totally absorbed in the body, with 95 percent absorption in 4 hours. T3 plays a crucial role in enhancing oxygen consumption, increasing basal metabolic rate, and influencing every organ system in the body. The biological half-life of T3 is about 2-1/2 days 2.

From the Research

T3 and Hypothyroidism

• T3, or triiodothyronine, is a hormone that plays a crucial role in the treatment of hypothyroidism, as evidenced by studies such as 3 and 4.
• The use of liothyronine (T3) in combination with levothyroxine (T4) has been explored as a potential treatment option for patients with hypothyroidism who remain symptomatic on T4 therapy alone, as discussed in 3 and 5.
• Research has shown that some patients may benefit from combination therapy with T4 and T3, particularly those who have not responded well to T4 monotherapy, as noted in 3 and 4.

Combination Therapy with T4 and T3

• Studies have investigated the use of combination therapy with T4 and T3, with some findings suggesting that this approach can be effective in restoring euthyroidism and improving symptoms, as reported in 3 and 6.
• However, other studies have noted that the benefits of combination therapy are not universally accepted, and more research is needed to fully understand its effects, as discussed in 5 and 4.
• The optimal dosing and timing of T3 administration in combination with T4 are still being explored, with some studies suggesting that a slow-release formulation of T3 may be beneficial, as mentioned in 4.

T3 Levels and Hypothyroidism

• Research has shown that patients with hypothyroidism may have lower serum T3 concentrations than the general population, even when treated with T4 monotherapy, as noted in 3 and 4.
• The use of T3 supplementation in combination with T4 may help to restore normal T3 levels and improve symptoms, as discussed in 3 and 4.
• However, the long-term consequences of combination therapy with T4 and T3 are not yet fully understood, and further research is needed to determine its safety and efficacy, as noted in 6.