Laboratory Studies

Routine initial evaluation of patients for hypo- or hyperthyroidism normally involves measuring blood levels of thyroid stimulating hormone (TSH), which is markedly elevated in hypothyroidism and markedly suppressed in hyperthyroidism. Initial evaluation may also include assay of free T4 in serum or, if free T4 assay is not locally available, assay of total T4 and T3 resin uptake and calculation of the free thyroxine index (FTI) which is proportional to the free T4 concentration.

In this case, since a firm, "lumpy" thyroid gland and a positive family history of thyroid disease raise the possibility of Hashimoto's thyroiditis, it would also be appropriate to test for anti-thyroid antibodies (anti-thyroglobulin and anti-microsomal), which are usually positive in Hashimoto's thyroiditis.

Initial Laboratory Studies

                               Patient           Reference
T4, Total (S)                 7.0 ug/dl           5 - 11.5
T3 resin uptake (S)            19%                 25 - 35
T3, Total (S)                 134 ng/dl          100 - 215
Free thyroxine index (FTI)    4.3                 6 - 11.5
TSH (S)                      22.0 uU/ml          0.7 - 7.0
Antithyroglobulin Ab.         Pos.  (1:640)         Neg.
Antimicrosomal Ab.            Pos.  (1:5120)        Neg.
Cholesterol (S)               230 mg/dl          140 - 225

(S = analyzed in serum specimens)

Questions

  1. What does the low T3 resin uptake mean and what might produce this result in a patient like this?
  2. How would you interpret these results?