Latest research on Levothyroxine

The major hormone derived from the thyroid gland. Thyroxine is synthesized via the iodination of tyrosines (monoiodotyrosine) and the coupling of iodotyrosines (diiodotyrosine) in the thyroglobulin. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. Thyroxine is peripherally deiodinated to form triiodothyronine which exerts a broad spectrum of stimulatory effects on cell metabolism. [PubChem]

Levothyroxine interactions

After exclusion of women on thyroid-related medications (Levothyroxine n=18, Propylthiouracil n=3) and women with overt biochemical hypothyroidism (n=10) or hyperthyroidism (n=1) on the blood samples, data from 956 women were included in this study. [source, 2016]
The Thyroid is absent, the hormone levels are low, the physician prescribes a dose of Levothyroxine to replace what the body would otherwise manufacture, and that resolves the problem. [source, 2016]
In the absence of a Thyroid gland, exogenous L-T4 is efficiently converted to T3. [source, 2016]
Serum T3 levels remain stable after L-T4 administration but vary widely after oral administration of liothyronine (L-T3). [source, 2016]
Several substances are known to interfere with intestinal absorption of L-T4, such as Cholestyramine, Aluminum hydroxide, Calcium carbonate, ferrous sulfate, and Sucralfate. [source, 2016]
Other substances can increase the hepatic metabolism of L-T4 (e.g. carbamazepine, Phenytoin, and Phenobarbital),16 whereas Estrogens such as oral contraceptives can increase TH requirements by increasing serum levels of T4-binding globulin.17 [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]
Jonklaas et al. showed that normal T3 levels can be achieved with traditional L-T4 therapy alone in patients who had undergone near-total or total thyroidectomy, which suggests that T3 administration is not necessary to Maintain serum T3 values at their endogenous pre-thyroidectomy levels.27 [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]
Suppression of TSH, using supraphysiological doses of L-T4, is used commonly to treat patients with Thyroid cancer in an effort to decrease the risk of recurrence.2–7 [source, 2016]