Levothyroxine (T4)

Overview

Levothyroxine (T4) is a synthetic form of thyroxine, the primary hormone secreted by the thyroid gland. T4 is largely inactive and serves as a prohormone, requiring peripheral conversion to triiodothyronine (T3) via deiodinase enzymes to exert biological effects. It is the most commonly prescribed thyroid medication worldwide (e.g., Euthyrox, Synthroid). Standard administration is on an empty stomach in the morning, as food and many supplements (calcium, iron, magnesium) significantly impair absorption. Conversion to T3 can be impaired by chronic stress, selenium deficiency, inflammation, or metabolic dysfunction, leading to excess reverse T3 (rT3) production. In such cases, T4 monotherapy may be insufficient. Novotyral 75/10 is a combination product containing both T4 (75 µg) and T3 (10 µg). Stack interactions: calcium, iron, magnesium, soy, and high-fiber supplements reduce absorption; must be taken separately.

Dosage

See product label · Empty stomach · Can take on empty stomach

Scientific Sources

• Thyroid Hormone Therapy for Older Adults with Subclinical Hypothyroidism. (N Engl J Med, 2017 Jun 29)
• Effects of Levothyroxine Treatment on Fertility and Pregnancy Outcomes in Subclinical Hypothyroidism. (Thyroid, 2024 Apr)
• Treatment of hypothyroidism with levothyroxine plus slow-release liothyronine. (Trials, 2025 Jul 1)
• Phase NA | UNKNOWN | Effect of Dietary Supplements on Levothyroxine Absorption in Pregnancy
• Phase PHASE4 | COMPLETED | Evaluation of Levothyroxine Formulation
• FDA approved | Synthroid (Levothyroxine) | NDA021402
• PubChem Compound 5819

Frequently Asked Questions