Prevalence of thyroid hormone alternations due to nonthyroidal illness, hypothyroidism and hyperthyroidism in sick patients
  In ambulatory and hospitalized patients, abnormal serum free T4 index and TSH values are more frequently due to nonthyroidal illnesses than to hypothyroidism or hyperthyroidism (3,5). The frequency of altered free T4 and free T3 estimates in sick sera appears to primarily depend upon the assay method (Figures 3 and 4) (5,9).


Figure 3:



Frequency of high or low free T4 estimates in patients with the low total T4 state of nonthyroidal illness.

Adapted from EM Kaptein (5).



Figure 4:



Estimates of free T3 by seven methods in patients with low total T4 and low total T3 levels due to nonthyroidal illnesses.

Data from Sapin et al. (9).



Serum TSH levels are transiently reduced to below normal values in 13% to 35% of "euthyroid" hospitalized patients with nonthyroidal illnesses (10,11). TSH values were below 0.1 mU/L (second generation assay) in 2% to 10% and below 0.01 mU/L (third generation assay) in up to 4% of these patients in the absence of hyperthyroidism, and did not correlate with alterations in free T4 estimates (10,11).

Although acute nonthyroidal illnesses frequently alter serum free T4 , free T3 and TSH values, these values normalize with recovery in euthyroid patients and are persistently abnormal in patients with concurrent thyroid diseases.