Background on Thyroid Hormones: T0, T1, T2, T3, T4
T0, T1 and T2 are hormone precursors and byproducts of thyroid hormone synthesis. They do not act on the thyroid hormone receptor and appear to be totally inert.
T4 is inert too, but conversion to T3 (in the liver, kidneys, brain, and other organs) activates it, in a highly regulated way.
Taking T3 (or using Armour thyroid) bypasses that natural regulation, which is generally an unhelpful thing to do.
Consequently, pure T4 is normally the best, most stable, and most reliable form of thyroid hormone replacement.
Although pure T3 is available as a pill or tablet, it is rarely used to treat hypothyroidism since the physiologic needs of the body are better served by the normal or gradual generation of T3 from T4. There are circumstances, however, when your endocrinologist may prescribe a T3 medication for brief periods of time; for example when preparing for radioactive iodine therapy, says the reference bookThyroid Cancer: A Guide for Patients.
The book also notes that “Professional organizations such as the American Association of Clinical Endocrinologists and the American Thyroid Association generally recommend that thyroxine (T4) alone (not in combination with other hormones) should be considered the mainstay for the therapy of all forms of hypothyroidism. This includes replacement thyroid hormone therapy for thyroid cancer.”
Dose adjustments may be necessary in pregnancy with increased doses early and throughout the pregnancy.
Changes in TSH (thyroid stimulating hormone) also can be observed when medications are added that influence absorption or metabolism of the thyroid hormone.
Drugs, foods, and conditions that may decrease T4 absorption.
Cholestyramine, cholestipol
Aluminum Hydroxide
Ferrous Sulfate
Cation-Exchange Resin
High Fiber Diet
Infants Fed Soybean Formula
Excess Soybean in Adults
Achlorhdria
H-2 Blockers
Jejunum-ileal Bypass Surgery
Antiseizure medications (e.g., phenobarbital, phenytoin, carbamazepine, and rifampin)
Sources: Thyroid Cancer: A Guide for Patients, 2nd edition, 2010, Chapters 2 and 29, and an additional ThyCa Medical Advisor.