Abstract
In euthyroidism, as thyroid Stimulating hormone (TSH) levels increase, the free triiodothyronine
(FT3) to free thyroxine (FT4) ratio increases. The aim of this study was to assess
if beyond the euthyroid range of TSH levels FT3/FT4 ratio continues to increase and
if levothyroxine treatment reduces this ratio, possibly through TSH suppression. This
cross sectional retrospective study included a total of 77 832 patients [age 22.76±15.17
years (4 days to 112 years)] evaluated and treated in community clinics between January
2009 and September 2013. Blood samples drawn in community clinics for which TSH, FT4,
FT3, age, and gender were available were included. Tests with TSH below 0.5 IU/l were
excluded as were samples taken during pregnancy. The FT3/FT4 ratio continued to increase
significantly even with TSH above 50 mIU/l (p for trend<0.001) with an increase of
more than 50% over the entire TSH range. With increasing age and female gender, the
phenomenon was less prominent (p<0.001). Levothyroxine treated patients had significantly
lower FT3/FT4 ratios in comparison to untreated patients up to TSH levels of 5.0 mIU/l.
In conclusion, increasing TSH increases FT3/FT4 ratio even with severe hypothyroidism,
less so with aging. With levothyroxine therapy, a ratio similar to untreated patients
is achieved at TSH of above 5.0 mIU/l. Since T3 suppresses TSH better than T4, administration
of T3 would likely normalize the FT3/FT4 ratio at a lower, ostensibly more physiological,
TSH level. This could be seen as a rationale for add-on T3 therapy.
Key words
thyroid function - thyroid hormones - thyrotropin
