Bioavailability of thyroid hormones from oral replacement preparations.

We evaluated gastrointestinal absorption in normal subjects of T4 and T3 from synthetic T3 tablets (Cytomel, SKF), desiccated thyroid tablets (Armour), thyroglobulin tablets (Proloid, Warner-Chilcott) and synthetic L-T4 tablets (Synthroid, Flint and Levothroid, Armour). Measurements of serum T4 and T3 concentrations and free hormone indices were made at multiple times after tablet ingestion, and T3 content in tablets was measured by radioimmunoassay. The time to peak serum T3, and the 26 hr intergrated increment in serum T3, Corrected for the amount if T3 ingested, were not significantly different for 75 micrograms of synthetic T3, 6 grains of desiccated thyroid (containing 99 micrograms T3) and 5 grains of thyroglobulin (containing 90 micrograms T3), the mean integrated increment values for the biological preparations being within 12% of those for synthetic T3. The peak serum T4 concentration, the time to peak T4, and 48 hr integrated increments in serum T4 and T3 were similar after 3 mg of Synthroid and Levothroid. The mean peak serum Free T3 Index after 75 micrograms T3, 500, was much higher than the mean peak Free T3 Index after 3 mg T4, 290. The time to peak Free T3 Index was much less after 75 micrograms T3, 2 hr, than the time to peak after 3 mg T4, 2 days. These results indicate that the time course and extent of T3 absorption do not differ, whether the T3 is given as the synthetic iodothyronine or as part of the thyroid protein, thyroglobulin. This approach appears to be useful in determining bioavailability of thyroid hormones from oral preparations and to assess the possibility of thyroid hormone malabsorption.