The hypothalamic-pituitary-thyroid axis in subjects with subclinical thyroid diseases: the impact of the negative feedback mechanism

To evaluate the hypothalamus-pituitary-thyroid (HPT) axis in patients with subclinical thyroid dysfunction recently reported to have either symptoms or organ involvements with potential morbidity, in order to better differentiate these patients with respect to controls. Sixtythree patients with subclinical hyperthyroidism (HyperT), 178 normal subjects (EuT) and 106 patients with subclinical hypothyroidism (HypoT) were enrolled; the groups presented normal thyroid hormone (FT4, FT3) levels and, respectively, reduced (HyperT), normal (EuT) and increased (HypoT) TSH levels. The negative feedback was simultaneously evaluated by multiple linear regression. The mean TSH, FT4 and FT3 levels were significantly different in the three groups. The negative correlation between thyroid hormones (FT4 and FT3) and TSH secretion was significant (p