TRH testing, T4-thyrotoxicosis and the aging thyroid gland.

Area of Science:

• Endocrinology
• Thyroidology
• Internal Medicine

Background:

• Elevated free thyroxine index (FT4I) can present diagnostic challenges.
• Differentiating between mild thyrotoxicosis and euthyroid states with abnormal FT4I requires accurate secondary thyroid function tests.

Purpose of the Study:

• To compare secondary thyroid function tests in patients with mildly thyrotoxicosis and euthyroid states presenting with an elevated FT4I.
• To determine the most reliable method for distinguishing these patient groups.

Main Methods:

• Comparison of secondary thyroid function tests.
• Intravenous thyrotropin-releasing hormone (TRH) test with serum TSH measurement at 20 minutes (delta TSH).
• Assessment of free T3 index and FT4I.

Main Results:

• A serum TSH measurement 20 minutes after intravenous TRH (delta TSH) most reliably separated mildly thyrotoxic from euthyroid patients.
• A significant delta TSH response (>0.5 microU/ml) helped exclude clinical thyrotoxicosis in patients with nodular goiter.
• The free T3 index was normal in one-third of mildly thyrotoxic patients and all euthyroid patients with falsely elevated FT4I.
• Blunted delta TSH responses in elderly women were linked to nodular goiter or occult thyroid nodularity.

Conclusions:

• The delta TSH response to TRH is a reliable indicator for differentiating mild thyrotoxicosis.
• This test is valuable in excluding thyrotoxicosis in patients with nodular goiter.
• Reduced TRH responses in the elderly may suggest partial thyroid autonomy rather than reduced thyrotroph synthetic capacity.