Thyroid Hormones-Thyrotropin Interrelationships in Thyroid Dysfunction States

Objective of this research is to explore the complex correlations among FT3, FT4 and TSH in thyroid dysfunction states. A total of 3425 outpatients were included to analyze their thyroid function tests measured at the university teaching hospital. The subjects under study were categorized as euthyroid (70.45%), subclinical hypothyroid (18.95%), overt hypothyroid (3.30%), subclinical hyperthyroid (5.11%) and overt hyperthyroid (2.19%) patients. The linear and non-linear relations among FT3, FT4 and TSH were modeled statistically to understand their role in thyroid functions. The prevalence of hypothyroidism is higher than hyperthyroidism in which females are more vulnerable than males. The correlation between FT3 and FT4 increases from euthyroidism (r = 0.1, P <0.001) to overt thyroid dysfunctions (r = 0.91, P <0.001). The non-linear complex correlations between FT4 and TSH are significantly explained by well fitted sigmoid curves of four-parameter logistic (4PL) model (R2 = 0.97 and P <0.001) in both hypothyroidism and hyperthyroidism interfaced with euthyroidism. The 4PL curves are response trajectories of pituitary TSH which exhibit the turning points of thyroid dysfunction states by the deviation in serum FT4. The negative slopes and correlations observed in the linear fits in TSH vs FT3 or FT4 are the indicators of negative feedback mechanism in the thyroid cycle.Competing Interest StatementThe authors have declared no competing interest.View Full Text