Bioavailability of thyroid hormones from oral replacement preparations

Area of Science:

• Endocrinology
• Pharmacokinetics
• Gastroenterology

Background:

• Thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are crucial for metabolism.
• Assessing gastrointestinal absorption of oral thyroid hormone preparations is vital for effective treatment.

Purpose of the Study:

• To compare the gastrointestinal absorption of T4 and T3 from synthetic and natural thyroid hormone preparations.
• To evaluate the bioavailability of T3 and T4 from different oral formulations.

Main Methods:

• Normal subjects ingested synthetic T3, desiccated thyroid, thyroglobulin, and synthetic L-T4 tablets.
• Serum T4 and T3 concentrations and free hormone indices were measured over time.
• T3 content in tablets was quantified using radioimmunoassay.

Main Results:

• Absorption of T3 from synthetic T3, desiccated thyroid, and thyroglobulin was comparable.
• Peak serum T3 levels and integrated T3 increments were similar across these preparations.
• Synthetic L-T4 and Levothyroid showed similar T4 and T3 absorption profiles.
• Peak Free T3 Index was higher and reached sooner with synthetic T3 compared to synthetic L-T4.

Conclusions:

• The time course and extent of T3 absorption are similar whether administered as synthetic iodothyronine or within thyroglobulin.
• The study methodology is useful for determining thyroid hormone bioavailability and assessing malabsorption.
• Different oral thyroid hormone preparations exhibit comparable T3 and T4 absorption in normal individuals.