Low Serum T4 Levels in Clinically Euthyroid Patients
A low T4 is common to all causes of hypothyroidism, but it is not absolutely diagnostic because a variety of severe acute and chronic illnesses, together with interference of drugs in thyroid function tests, may also cause low T4 values (Fig. 15.34). Thus, low TBG levels or competitive binding by drugs alters the binding capacity, and total T4 levels are reduced, but free T4 levels are usually normal. Even when TBG levels are not affected, severe illness may lower T4 production and T4 to T3 conversion. rT3 levels are usually increased, and TSH levels are normal. In asymptomatic autoimmune thyroiditis, total T4 levels may be towards the lower limit of normal, but TSH levels are often mildly elevated.
