[Does the determination of basal TSH level replace the TRH test?]

Area of Science:

• Endocrinology
• Clinical Chemistry

Background:

• Supersensitive thyrotropin (TSH) assays enable detection of very low concentrations.
• Manifest hyperthyroidism typically presents with low TSH levels and minimal response to TRH stimulation.
• Previous claims suggested basal TSH estimation is sufficient for hyperthyroidism screening.

Purpose of the Study:

• To evaluate the reliability of basal TSH levels as a sole screening test for hyperthyroidism.
• To determine reference intervals for basal TSH and TSH response to TRH stimulation.
• To assess the correlation between basal TSH and TRH-stimulated TSH response.

Main Methods:

• Determination of reference intervals for basal TSH and TRH-stimulated TSH.
• Statistical analysis of basal TSH and TSH response to TRH stimulation.
• Correlation analysis between basal TSH and TSH response to TRH stimulation.

Main Results:

• The reference interval for healthy individuals' basal TSH is 0.25-4.0 mU/l.
• The reference interval for TSH response to TRH stimulation is 3.0-20 mU/l.
• A poor correlation (r=0.83) exists between basal TSH and TRH-stimulated TSH, with broad overlapping zones in reactivity histograms.

Conclusions:

• Basal TSH levels alone are not sufficient for reliable hyperthyroidism exclusion.
• A significant "gray zone" exists where prediction of TRH response is unreliable based solely on laboratory data.
• Clinical findings are essential for accurate diagnosis in the indeterminate laboratory ranges.