Thyroid-Binding Globulin (TBG)
Thyroid-binding globulin is the main carrier protein that moves thyroid hormone through the blood. Almost all thyroid hormone travels bound to a carrier. Less than one tenth of one percent of T4 floats free, and only the free part enters cells.1
This is why a total reading can mislead. Estrogen raises carrier protein levels, so pregnancy and estrogen medicines push the total up. In pregnancy the free level shifts too, so ranges are set by trimester. Thyroid function touches ovulation, the luteal phase and early pregnancy, so a whole-picture evaluation includes it.
Other hormones move the free share as well. In a 12-week randomized trial in postmenopausal women, oral micronized progesterone raised free T4 compared with placebo.2 Free levels and TSH carry information a total cannot.
Cited in this entry
- Kelly GS. Peripheral metabolism of thyroid hormones: a review. Alternative Medicine Review : a Journal of Clinical Therapeutic. 2000. https://rrmacademy.org/library/peripheral-metabolism-of-thyroid-hormones-a-review-recaxvxkumcn2mog6/
- Sathi P, Kalyan S, Hitchcock CL, Pudek M, Prior JC. Progesterone therapy increases free thyroxine levels--data from a randomized placebo-controlled 12-week hot flush trial. Clinical Endocrinology. 2013. https://pubmed.ncbi.nlm.nih.gov/23252963/
This content is for educational purposes only and does not constitute medical advice. Consult an RRM clinician or healthcare provider for guidance specific to your situation.