This 2016 research study discovered that TSH plays a role in enhancing T4-T3 conversion, but TSH is less able to enhance T3 levels after age 40.
Free T3, free T4, and TSH levels from 527, 564 sera collected from patients aged 1 year or greater were studied. … 27,940 samples remaining after exclusion were stratified by age. Correlations of TSH to FT4, FT3, and FT3/FT4 ratios by age group were examined.
Up to age 40, for each increasing TSH quartile, FT3 and the FT3/FT4 ratio increased and FT4 decreased significantly (for both FT3, FT4 and FT3/FT4 ratio, P<0.05 for every TSH quartile when compared with the 1st quartile, except FT3 in the 30-40 age group).
In older age groups, increasing TSH was not associated with increased FT3/FT4 ratio.
As TSH levels increase, FT3/FT4 ratios increase until age 40, but this differential increase does not occur in older age groups.
This may reflect a decrease in thyroxine (T4) to triiodothyronine (T3) conversion with age, which may be part of the aging process.
Strich, D., Karavani, G., Edri, S., & Gillis, D. (2016). TSH enhancement of FT4 to FT3 conversion is age dependent. European Journal of Endocrinology, 175(1), 49–54. LINK: https://www.ncbi.nlm.nih.gov/pubmed/27150496
This means that mature adults and seniors get less “bang for their buck” out of their TSH.
Ageism affects thyroid testing and thyroid therapy if Free T3 levels are not tested and lower levels of Free T3 are not treated.
The T3-T4 ratio plays a role in discovering reduced thyroid hormone conversion.