Levothyroxine (T4) is the main secretory product of the thyroid gland and the most abundant circulating thyroid hormone. T4 itself has minimal thyroid receptor affinity — it is a prohormone. Peripheral deiodination by type 1 and type 2 deiodinase (DIO1, DIO2) converts T4 to the active T3 in liver, kidney, and other tissues. Approximately 20% converts to reverse T3 (rT3, inactive). The T4→T3 conversion rate determines effective T3 availability — providing a research model for studying deiodinase regulation, selenium status (deiodinase cofactor), and tissue-specific T3 generation. Used as the clinical standard reference for thyroid hormone research.
Also Known As
Levothyroxine, Synthroid, Eltroxin, Thyroxine
Compound Class
Thyroid hormone — prohormone (converted to active T3)
Molecular Formula
C₁₅H₁₁I₄NO₄
Molecular Weight
776.87 Da
CAS Number
51-48-9
Half-Life
~7 days
Research Applications
Deiodinase / T4→T3 Conversion Research
T4 peripheral deiodination to T3 is regulated by DIO1 (liver, kidney) and DIO2 (CNS, pituitary). Selenium-dependent enzymatic process. T4 is the substrate for studying conversion efficiency, tissue-specific T3 generation, and reverse T3 formation under stress or illness.
Primary Hypothyroidism Models
T4 is the reference replacement therapy in thyroidectomised or PTU-treated hypothyroid rodent models. HPT axis suppression, TSH normalisation, and tissue T3 restoration studies use T4 as the gold standard replacement compound.
HPT Axis / TSH Feedback Research
T4 → T3 conversion in pituitary → TRβ2 activation → TSH suppression. T4 is the dominant HPT axis regulator. Used in TSH feedback dose-response, pituitary TRβ2 selectivity, and thyroid axis regulation research.
Combination T4+T3 vs T4 Monotherapy
Research interest in whether T4 monotherapy achieves adequate tissue T3 — combination T4+T3 protocols vs T4 alone in hypothyroidism models. DIO2 polymorphism subset analysis. Emerging area with significant clinical translation.
Key Research Data
Study
Key Findings
Bunevicius et al. 1999 NEJM
T4+T3 combination vs T4 alone in hypothyroid patients: cognitive and mood improvements with combination
Bianco 2011 JCEM Review
Comprehensive DIO2 polymorphism and T4→T3 conversion — definitive mechanistic reference
Thyroid hormone receptor selectivity
T4 vs T3 TR binding: T4 Ki ~100× lower affinity than T3 — prohormone status confirmed across isoforms
T4 (Levothyroxine) is a Thyroid hormone — prohormone (converted to active T3). Levothyroxine (T4) is the main secretory product of the thyroid gland and the most abundant circulating thyroid hormone. T4 itself has minimal thyroid receptor affinity — it is a prohormone. Peripheral deiodination by type 1 and type 2 deiodinase (DIO1, DIO2) …
What research applications does T4 support?
T4 peripheral deiodination to T3 is regulated by DIO1 (liver, kidney) and DIO2 (CNS, pituitary). Selenium-dependent enzymatic process. T4 is the substrate for studying conversion efficiency, tissue-specific T3 generation, and reverse T3 formation under stress or illness….
How does T4 compare to similar compounds?
T4 is the reference replacement therapy in thyroidectomised or PTU-treated hypothyroid rodent models. HPT axis suppression, TSH normalisation, and tissue T3 restoration studies use T4 as the gold standard replacement compound….
Does QSC ship T4 to the USA?
Yes. QSC ships domestically across the USA and to EU, UK, Canada, and Australia. Every batch carries a Janoshik third-party COA confirming ≥99% HPLC purity and MS identity.
