Levothyroxine (T4)

Also known as: levothyroxine sodium, L-thyroxine, T4, Synthroid (branded)

Hormone Restoration

In plain English

Levothyroxine is a lab-made copy of the main thyroid hormone (T4). It treats an underactive thyroid (hypothyroidism), restoring energy, temperature regulation, and metabolism. It is taken once daily on an empty stomach, and the dose is adjusted using blood tests. It is the standard, well-established treatment for low thyroid.

The science

Levothyroxine is synthetic thyroxine (T4), a prohormone converted peripherally by deiodinases to the more active triiodothyronine (T3); it restores euthyroid status and normalizes TSH. The American Thyroid Association (Jonklaas 2014) recommends levothyroxine monotherapy as the standard of care, titrated to TSH, and found no consistently strong evidence that combination T4/T3 or thyroid extract is superior. FDA-approved levothyroxine products are available and are considered narrow-therapeutic-index drugs where consistency of formulation matters; over-replacement risks include atrial fibrillation and accelerated bone loss, while under-replacement leaves symptoms uncontrolled. Absorption is affected by food, calcium, iron, and PPIs. Compounded levothyroxine (including sustained-release capsules) is not an FDA-approved formulation and adds variability to a drug that requires tight dose control.

References

• Jonklaas et al., Thyroid 2014 (ATA hypothyroidism treatment guidelines)