Abstract
BackgroundBasal 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.MethodsA 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.ResultsBasal 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.ConclusionOur 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.
Highlights
Basal thyroid stimulating hormone (TSH) levels reflect the metabolic status of thyroid function, the definition and interpretation of the basal levels of TSH is a matter of controversial debate
Our analysis strategy allows us to delineate the predictive value of basal TSH levels in relation to latent hypothyroidism
Analysis of fT3 and fT4 and of basal TSH levels Stratification of data according to age groups did not reveal any age-dependency of thyroid function parameters
Summary
Basal TSH levels reflect the metabolic status of thyroid function, 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. In order to correctly identify these patients it is imperative to have a clear definition of the upper reference range for basal TSH. Patients whose TSH lies in the upper reference range might appear to have minimal thyroid deficiency. This might appear to be an easy task, the definition of the upper reference range for TSH (page number not for citation purposes). An alternative approach is to rely on the physiological response of TSH to TRH stimulation [22] taking the absolute TSH values 30 minutes after TRH stimulation as the classification criteria [17,23,24]
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