The relationship between serum TSH and free T4 in older people.

The frequency distribution of serum thyroid stimulating hormone (TSH) shows a skewed pattern that may change with age. The set point of the hypothalamic-pituitary-thyroid axis for an individual is thought to be genetically determined and has been described as a log-linear relationship of serum TSH to free thyroxine (T4); however, the validity of this hypothesis has yet to be established in older people. The aim of the study was to describe the relationship between serum TSH and free T4 in older people and define factors influencing this relationship. We conducted a cross-sectional, observational study of thyroid function in a community population of older subjects over 65 years of age. The relationship between serum TSH and free T4 was not linear as previously described, but is best described as a fourth-order polynomial. Both gender and smoking status affected the relationship. This suggests that more complex modelling is required when investigating the hypothalamic-pituitary-thyroid axis.