The diagnosis of hypothyroidism and hyperthyroidism during pregnancy employs the same laboratory tests and utilizes the same principles discussed above. Pregnancy is associated with an increase in levels of serum total T4 and serum T3 caused principally by a rise in the concentration of thyroxine-binding globulin, the predominant iodothyronine-binding protein in serum. Serum free T4 concentrations should be normal in patients with increased thyroxine-binding globulin levels alone. Despite these changes, serum TSH concentration by sensitive assay is normal in pregnant women without thyroid disease.[25] Thus, hyperthyroidism in pregnant women can usually be diagnosed by an abnormally high level of serum free T4 and suppressed serum TSH. Hypothyroidism can be documented by decreased serum free T4 and increased serum TSH levels.
