Changes in laboratory indicators of thyroid function in non-thyroid diseases are called thyroid pseudodysfunction.
There are two options for pseudodysfunction of the thyroid gland:
– Pseudodysfunction of the thyroid gland with low T4 . In this case, changes in the levels of T3, T4 and TSH and the index of binding of thyroid hormones are characteristic.
– Pseudodysfunction of the thyroid gland with high T4 . A temporary increase in total T4 is observed with many non-thyroid diseases, especially with relatively mild ones. Usually, after a few days, total T4 normalizes without treatment.
T3 . In severe non-thyroid diseases, serum levels of total and free T3 decrease due to suppression of peripheral 5′-monodeiodination of T4 . The conversion of T4 to T3 is impaired in diseases of the liver , uremia , severe infections , DKA , surgical interventions , starvation , burns , and extensive myocardial infarction .
T4 . In patients in serious condition , the level of total T4 decreases , and the level of free T4 can remain normal or also decrease. The basal TSH level does not change. Such hormonal changes are regarded as transient secondary hypothyroidism . It is believed that this is an adaptive reaction to increased catabolism . During the recovery period, the TSH level rises (sometimes up to 10-20 mU / l); T3 and T4 levels normalize.