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.

Background: The thyroid status is evaluated by two clinical diagnostic tests which are thyroid ultrasonography and thyroid function tests. The objective of this research is to critically analyze the age and gender based variations of thyroid volume and thyroid hormone levels in the hospital based euthyroid subjects. Methodology: A total of 221 euthyroid subjects aged 1-86 years were selected to observe the normal thyroid size by ultrasonography at Department of Radiology and the thyroid function test values (FT3, FT4 and TSH) of 2413 euthyroid subjects aged (<1)-93 years were observed at Department of Biochemistry, Tribhuvan University Teaching Hospital, Kathmandu, Nepal during January 2017 to February 2018. The observed data were analyzed graphically and statistically to check the cross-correlations among the variables. Results: The best fitted equations with significant correlation coefficients and p < 0.05 provide the empirical relations between any two of the observed variables: age, thyroid lobe volumes, FT3, FT4 and TSH. The mean ± SD (p < 0.0001) of thyroid volume, FT3, FT4 and TSH are 4.74 ± 2.30 mL, 5.46 ± 0.82 pmol/L, 14.09 ± 2.71 pmol/L and 2.30 ± 0.98 mIU/L, respectively. Conclusion: The thyroid size first increases and then decreases whereas the thyroxin level first decreases and then increases with aging. Left lobe volume is almost same for both genders and right lobe volume is higher in males. The thyroid size in menarche and menopause periods of females is larger than that of males. Such age-and gender-specific changes recommend the new reference ranges for the normal thyroid functions.

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