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Thyroid iodide compartments and their implication in the rat thyroid iodine organification
Endocrinology
|May 1, 1982
Summary
Transported iodide directly iodinates new thyroglobulin (Tg), while internal iodide iodinates existing Tg. Hormone secretion primarily uses older iodine stores, indicating distinct Tg compartments and TSH-dependent processes.
Area of Science:
- Endocrinology
- Thyroid Physiology
- Iodine Metabolism
Background:
- Thyroglobulin (Tg) iodination is crucial for thyroid hormone synthesis.
- The relative contributions of circulating and internally sourced iodide remain unclear.
- Thyroid-stimulating hormone (TSH) influences thyroid function and hormone production.
Purpose of the Study:
- To differentiate the roles of transported and internal iodide in thyroglobulin iodination.
- To investigate the dynamics of iodine incorporation into Tg and subsequent hormone release.
- To elucidate the impact of TSH on these processes.
Main Methods:
- Radioactive iodide injection in intact and hypophysectomized rats.
- Measurement of specific radioactivities (SRAs) in thyroid iodide, Tg, lysosomal iodine, and plasma hormones over 92 hours.
- Analysis of iodine metabolism and Tg compartments.
Main Results:
- In control rats, Tg and lysosomal iodine peaked at 12h, while hormonal iodine peaked later at 48h.
- Lysosomal iodide SRA was lower than Tg SRA, suggesting mixing of old and new iodine.
- Hypophysectomized rats showed a single peak at 24h for Tg, lysosomes, and hormones, with similar SRAs, indicating reduced old iodine compartments.
Conclusions:
- Transported iodide primarily iodinates newly synthesized Tg, independent of TSH.
- Internal iodide mainly contributes to delayed iodination of existing Tg, a TSH-dependent process.
- Evidence supports at least two Tg compartments with differing turnover rates and hormone content.