Hyperthyroidism
34
Box 5. Considerations of (F)T3 Testing in Gestational
Hyperthyroidism
In the case of suspected hyperthyroidism in pregnancy, several pertinent issues should
be noted when considering the measurement of serum (F)T3.
• Analytic concerns of the free thyroid hormone assays in pregnancy, particularly for
FT3, may limit their usefulness (Definition of (Ab)normal Thyroid Function Tests).
Compared to total triiodothyronine (T3), FT3 is not altered by binding proteins and
appears to be more stable throughout pregnancy, although FT3 may be not as widely
available as total T3. When T3 estimates are needed, it is reasonable to apply the same
considerations as described for free and total T4 in Section C, while noting the general
limitations of the FT3 assay similar to its concerns in nonpregnant individuals.
• In general, serum (F)T3 testing can be performed in cases of a suppressed TSH and
normal FT4, to distinguish subclinical hyperthyroidism from overt hyperthyroidism,
especially if this would change management.
• If T3 thyrotoxicosis is suspected, for example hyperthyroidism caused by Graves'
disease or a T3-producing thyroid nodule, T3 testing may be considered in a pregnant
woman with a normal FT4 and a TSH <0.01 mU/L, or TSH ranging from >0.01 to
<0.3 mU/L in the presence of thyrotoxic symptoms, thyroid eye disease, a palpable
thyroid nodule, and absent use of ATDs or thyroid hormone medication.
In general, serum T3 testing may be helpful to initially distinguish the etiolog y of
hyperthyroidism during pregnancy and can be used to guide the diagnosis between
gestational transient thyrotoxicosis (GTT), Graves' disease (Table 2), and a T3-
producing thyroid nodule. However, maternal T3 concentrations are not associated
with fetal T3 or (F)T4, and as such, targeting a normal (F)T3 during pregnancy can
cause fetal hypothyroidism. Therefore, (F)T3 should not be used to monitor or inform
medical management for GTT or Graves' disease during pregnancy.
