Why taking thyroid pills feels different than having an overactive thyroid
Heterogenous biochemical expression of hormone activity in subclinical/overt hyperthyroidism and exogenous thyrotoxicosis
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Surprising Findings
FT3 levels were nearly identical between pill users and people with overactive thyroids, despite FT4 being 30% higher in pill users.
Common belief: More T4 = more T3. But here, the body’s conversion system shuts down in pill users, so T3 doesn’t rise—even with excess T4. This flips the script on how we think hormone metabolism works.
Practical Takeaways
If you're on levothyroxine and still feel tired or gaining weight, ask your doctor to check FT4, FT3, and deiodinase activity—not just TSH.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Surprising Findings
FT3 levels were nearly identical between pill users and people with overactive thyroids, despite FT4 being 30% higher in pill users.
Common belief: More T4 = more T3. But here, the body’s conversion system shuts down in pill users, so T3 doesn’t rise—even with excess T4. This flips the script on how we think hormone metabolism works.
Practical Takeaways
If you're on levothyroxine and still feel tired or gaining weight, ask your doctor to check FT4, FT3, and deiodinase activity—not just TSH.
Publication
Journal
Journal of Clinical & Translational Endocrinology
Year
2020
Authors
R. Hoermann, J. Midgley, R. Larisch,, Johannes W. Dietrich
Related Content
Claims (7)
Giving synthetic thyroid hormone externally raises levels of thyroid hormone in the blood and makes an existing overactive thyroid condition more severe.
People taking synthetic thyroid hormone after thyroid removal have higher levels of FT4 but similar levels of FT3 compared to people with overactive thyroid glands due to Graves' disease or toxic adenoma, suggesting different ways the body processes these two thyroid hormones.
In people with thyrotoxicosis caused by taking too much thyroid hormone medication, the brain's control system for thyroid hormone levels behaves differently than in people whose thyroid gland overproduces hormones on its own. Specifically, TSH stays low even when thyroid hormone levels are higher than normal.
People taking synthetic thyroid hormone (levothyroxine) for thyroid conditions show lower levels of a specific enzyme activity that converts T4 to T3 in the body, compared to people whose thyroid glands overproduce hormones due to Graves' disease or a toxic nodule.
People taking levothyroxine medication for thyroid conditions tend to have a higher body mass index than those whose thyroid overproduces hormones naturally, even when their blood levels of active thyroid hormone are similar. This suggests that the source of excess thyroid hormone may influence body weight differently.