The Human thyroid gland is a major component of the endocrine system. Thyroid hormones perform
many important functions. They exert powerful and essential regulatory influences on growth,
differentiation, cellular metabolism, and general hormonal balance of the body, as well as on the
maintenance of metabolic activity and the development of the skeletal and organ system.
The hormones thyroxine (T4) and 3,5,3’ thiiodothyronine (T3) circulate in the bloodstream, mostly
bound to the plasma protein, thyroxine binding globulin (TBG). The concentration of T3 is much less
than that of T-4, but its metabolic potency is much greater.
T3 determinations an important factor in the diagnosis of thyroid disease. Its measurement has
uncovered a variant of hyperthyroidism in thyrotoxic patients with elevated T3 values and normal T4
values. An increase in T3 without an increase in T4 is frequently a forerunner of recurrent
thyrotoxicosis in previously treated patients. The clinical significance of T3 is also evident in patients in
whom euthyroidism is attributable only to normal T3, although their T4 values are subnormal.
T3 determination is also useful in monitoring both patients under treatment for hyperthroidism and
patients who have discontinued anti-thyroid drug therapy. It is especially valuable in distinguishing
between euthyroid and hyperthyroid subjects.
In addition to hyperthyroidism, T3 levels are elevated in women who are pregnant, and in women
receiving oral contraceptives or estrogen treatment, paralleling TBG increases in a manner analogous
to T4 levels. Likewise, a reduction in TBG concentration decreases T3 concentration. These changes
in the T3 level, however, are not a true reflection of thyroid status.