TSH, FT4 and FT3 have individual, but interlocking roles in thyroid homeostasis

What is it about?

This study investigated patterns of thyroid hormones in 1912 subjects with different thyroid conditions. It revealed that correlations between FT3, FT4 and TSH were modulated by multiple factors including age, body mass, thyroid volume, antibody status and L-T4 treatment. FT3 levels and calculated deiodinase activity (SPINA-GD) were significantly lower in subjects treated with levothyroxine.

Featured Image

Why is it important?

Equilibria typical of the healthy state are not invariant, but profoundly altered by multiple factors, e.g. L-T4 treatment. This study suggests that strategies for optimising treatment should be revisited. It delivered new evidence that T3 levels are lower in patients on substitution therapy with levothyroxine. In addition, the time point of taking levothyroxine, i.e. before or after blood drawing, was revealed to be of less influence on lab results than expected.