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Release of thyroid hormone from circulating thyroglobulin in the rat
Acta Endocrinologica
|February 1, 1986
Summary
Circulating thyroglobulin (Tg) is broken down in tissues outside the thyroid, releasing thyroid hormones. However, the study found these released thyroid hormones (T4 and T3) are not significant enough to impact physiology.
Area of Science:
- Endocrinology
- Molecular Biology
- Biochemistry
Background:
- Thyroglobulin (Tg) is present in peripheral blood, but its fate is unknown.
- Investigating the extrathyroidal metabolism of circulating Tg is crucial for understanding thyroid hormone regulation.
Purpose of the Study:
- To determine if circulating thyroglobulin (Tg) is hydrolyzed in extrathyroidal tissues.
- To ascertain if thyroid hormones are released from circulating Tg in peripheral blood.
Main Methods:
- In vivo radiolabeling of rat thyroglobulin (Tg) with 125I.
- Intravenous injection of labeled Tg into rats with blocked thyroids (KI).
- Analysis of plasma samples using ion-exchange chromatography and Sephadex G-25 gel filtration.
Main Results:
- Radioactivity was recovered in iodoprotein, iodoaminoacid, and iodide fractions over time.
- Significant proportions of radioactivity were found in T4 and T3 within the iodoaminoacid fraction after 20 hours.
- Over 75% of the iodoaminoacid radioactivity was incorporated into T4 and T3 at 20 hours post-injection.
Conclusions:
- Circulating thyroglobulin (Tg) undergoes hydrolysis in extrathyroidal tissues.
- Thyroid hormones (T4 and T3) are released into circulation from Tg breakdown.
- The quantity of thyroid hormones released extrathyroidally is not physiologically significant.