Increased thyroidal T4 to T3 conversion in autonomously functioning thyroid adenoma: from euthyroidism to...

Area of Science:

• Endocrinology
• Thyroidology
• Molecular Endocrinology

Background:

• Autonomously functioning thyroid adenoma (AFTA) is characterized by TSH-suppressed serum levels.
• The role of intrathyroidal T4 to T3 conversion in AFTA pathogenesis remains incompletely understood, particularly in borderline thyroid function.
• Investigating local hormone conversion can elucidate disease mechanisms.

Purpose of the Study:

• To determine if intrathyroidal conversion of T4 to T3 within AFTA tissue influences serum T3 levels and TSH suppression.
• To assess this conversion in patients with normal T4 but suppressed TSH, including those with borderline T3 levels.

Main Methods:

• Ten patients with AFTA and suppressed TSH were studied.
• Thyroidal conversion of T4 to T3 was measured in surgically removed AFTA and adjacent TSH-suppressed tissue.
• Enzymatic conversion assays involved incubating T4 with tissue homogenates and measuring T3 generation over time.

Main Results:

• T3 production was significantly higher in AFTA tissue compared to paranodular tissue (8.8 vs. 1.8 pmol/mg protein, p<0.01).
• Four patients with borderline or elevated serum T3 exhibited excessively high T3 generation in AFTA tissue.
• A significant correlation was observed between serum T3 concentrations and the rate of T3 generation from T4 in AFTA tissue.

Conclusions:

• Increased intrathyroidal T4 to T3 conversion in AFTA tissue contributes to elevated serum T3 levels.
• This enhanced conversion may play a role in TSH suppression, particularly in the transition from euthyroid to early hyperthyroid states.
• AFTA tissue's heightened enzymatic activity is a key factor in local and systemic thyroid hormone regulation.