Latest research on Triiodothyronine

The L-triiodothyronine (T3, liothyronine) thyroid hormone is normally synthesized and secreted by the thyroid gland in much smaller quantities than L-tetraiodothyronine (T4, levothyroxine, L-thyroxine). Most T3 is derived from peripheral monodeiodination of T4 at the 5 position of the outer ring of the iodothyronine nucleus. The hormone finally delivered and used by the tissues is mainly T3. [PubChem]

Latest findings

Isolated hypothyroxinaemia has also been shown to be associated with gestational diabetes (7), and a recent study found that lower free thyroxine (FT4) levels and higher free Triiodothyronine (FT3) to FT4 ratios (suggesting higher peripheral deiodinase activity leading to conversion of FT4 to FT3) in pregnant women are associated with several adverse metabolic parameters relating to obesity, glycaemia, insulin resistance and lipid profile (9). [source, 2016]
The Thyroid gland secretes both thyroxine (T4) and Triiodothyronine (T3), which exert a negative feedback on TSH releasing hormone and TSH secretion.10,12 [source, 2016]
Epithelial cells were then cultured in a keratinocyte culture medium composed of DMEM/F-12, supplemented with 10 ng/mL epidermal growth factor (Sino Biological), 10 μg/mL insulin (Wako), 0.5 μg/mL Hydrocortisone (Wako), 0.25 μg/mL Isoproterenol (Wako), 1.3 ng/mL Triiodothyronine (MP Biomedicals), 100 units/mL penicillin, 100 μg/mL STREPTOMYCIN, 0.25 μg/mL Amphotericin B, and 4% fetal bovine serum (FBS, Gibco) throughout a two-week culture period. [source, 2016]
Serum T3 levels remain stable after L-T4 administration but vary widely after oral administration of liothyronine (L-T3). [source, 2016]
Because of these characteristics, L-T3 administration results in widely variable serum levels, making it a less appropriate form of Thyroid hormone replacement.14,18 [source, 2016]
L-T3 tablets are commercially available in the United States as Cytomel. [source, 2016]
Therapy with synthetic L-T3 has the theoretical advantage of bypassing the T4-to-T3 conversion step; however, the ability of the body’s tissues to use this T3 is not clear, and therefore there is no physiologic basis for its use. [source, 2016]
Futhermore, direct treatment with L-T3 also has the disadvantage of not permitting regulated time-specific and tissue-specific production of T3 through peripheral deiodination of T4. [source, 2016]
With T3 monotherapy, multiple daily dosing is required to sustain serum T3 levels due to the shorter half-life of L-T3 compared to L-T4.26 [source, 2016]
If normalization of serum TSH is the goal of L-T3 monotherapy, serum T3 levels must be significantly higher than (approximately double) those seen during L-T4 monotherapy; whether this leads to relative hyperthyroidism in some tissues requires further study. [source, 2016]