Diagnostic accuracy of basal TSH determinations based on the intravenous TRH stimulation test: An evaluation of 2570 tests and comparison with the literature

Helga Moncayo¹^,3 , Otto Dapunt¹^,3 and Roy Moncayo¹^,2^,3

¹Department of Obstetrics and Gynecology, University of Innsbruck, Anichstrasse 35, A-6020 Innsbruck, Austria

²Department of Nuclear Medicine, Medical University Innsbruck, Austria

³WOMED, Karl-Kapferer-Strasse 5, 6020 Innsbruck, Austria

author email corresponding author email

BMC Endocrine Disorders 2007, 7:5doi:10.1186/1472-6823-7-5


Published:	2 August 2007

Abstract

Background

Basal TSH levels reflect the metabolic status of thyroid function, however the definition and interpretation of the basal levels of TSH is a matter of controversial debate. The aim of this study was to evaluate basal TSH levels in relation to the physiological response to i.v. TRH stimulation.

Methods

A series of 2570 women attending a specialized endocrine unit were evaluated. A standardized i.v. TRH stimulation test was carried out by applying 200 μg of TRH. TSH levels were measured both in the basal and the 30 minute blood sample. The normal response to TRH stimulation had been previously determined to be an absolute value lying between 2.5 and 20 mIU/l. Both TSH values were analyzed by cross tabulation. In addition the results were compared to reference values taken from the literature.

Results

Basal TSH values were within the normal range (0.3 to 3.5 mIU/l) in 91,5% of cases, diminished in 3,8% and elevated in 4.7%. Based on the response to TRH, 82.4% were considered euthyroid, 3.3% were latent hyperthyroid, and 14.3% were latent hypothyroid. Combining the data on basal and stimulated TSH levels, latent hypothyroidism was found in the following proportions for different TSH levels: 5.4% for TSH < 2.0 mIU/l, 30.2% for TSH between 2.0 and 3.0 mIU/l, 65,5% for TSH between 3.0 and 3.50 mIU/l, 87.5% for TSH between 3.5 and 4.0 mIU/l, and 88.2% for TSH between 4 and 5 mIU/l. The use of an upper normal range for TSH of 2.5 mIU/l, as recommended in the literature, misclassified 7.7% of euthyroid cases.

Conclusion

Our analysis strategy allows us to delineate the predictive value of basal TSH levels in relation to latent hypothyroidism. A grey area can be identified for values between 3.0 and 3.5 mIU/l.