Physiological aspects of the thyroid trapping function and its suppression in iodine deficiency using 99mTc...

Area of Science:

• Endocrinology
• Nuclear Medicine
• Thyroidology

Background:

• Iodine deficiency is a significant public health concern, impacting thyroid function.
• Euthyroid endemic goitre is common in iodine-deficient areas.
• Thyroid scintigraphy is a key diagnostic tool for assessing thyroid function.

Purpose of the Study:

• To establish reference ranges for thyroid uptake of 99mTc pertechnetate (iodide trapping function) in individuals without thyroid autonomy.
• To investigate factors influencing thyroid iodide trapping function in an iodine-deficient population.
• To assess the relationship between thyroid trapping function and TSH levels.

Main Methods:

• Quantitative scintigraphy was used to measure 99mTc pertechnetate uptake before (TcUb) and under suppression (TcUs) in 190 individuals.
• Thyroid Stimulating Hormone (TSH) response to Thyrotropin-Releasing Hormone (TRH) was assessed.
• Factors such as thyroid weight, age, sex, and iodine status were analyzed for their influence on trapping function.

Main Results:

• Reference ranges for TcUb and TcUs were determined as 7.4% and 1.6%, respectively.
• Thyroid iodide trapping increased with decreased iodine supply, demonstrating a compensatory mechanism.
• Trapping function correlated positively with thyroid weight and inversely with delta TSH after TRH stimulation.

Conclusions:

• Thyroid follicular cells adapt their iodide trapping function to iodine availability, with TSH having a secondary role.
• The reference range for TcUs is crucial for accurately detecting thyroid autonomy in vivo.
• A continuum of TSH-independent thyroid function exists even in individuals without overt autonomy.