What is the role of lab testing in the diagnosis of amiodarone-associated thyroid dysfunction?

Updated: Aug 28, 2020
  • Author: Mini Gopalan, MD; Chief Editor: Romesh Khardori, MD, PhD, FACP  more...
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Lab findings for AIH are similar to those for spontaneous hypothyroidism and include decreased levels of serum free T4 and increased levels of serum TSH. Serum thyroglobulin levels are often increased, probably because of TSH-enhanced thyroid stimulation.

Lab findings for AIT are elevated levels of serum total and serum free T4 and T3, and undetectable levels of TSH. Low TSH levels and elevated free T4 levels are also commonly seen in the early phases of amiodarone therapy and in patients with severe nonthyroidal illness who have euthyroidism and are treated with amiodarone. Therefore, the measurement of free T3 levels may be helpful in differentiating conditions, because free T3 levels are increased in hyperthyroidism, while they are decreased in early phases of treatment with amiodarone. Serum rT3 levels are also markedly increased. However, serum rT3 levels are not part of a routine workup.

Because amiodarone has no effect on the serum concentration of thyroid hormone-binding globulin, changes in the levels of free T4 and free T3 mirror those for total T4 and total T3.

In the absence of hypothyroid symptoms, moderately elevated serum TSH levels with high normal or raised serum free T4 levels may reflect subclinical hypothyroidism. Close monitoring and repeat testing after 6 weeks is recommended.

Serum sex hormone–binding globulin concentration is increased in patients with AIT but not in patients with hyperthyroxinemia and euthyroidism who are treated with amiodarone therapy. This assay is of limited importance, however, because of the numerous factors that affect the serum levels.

Serum thyroglobulin levels are not diagnostic because they are usually higher in type 2 AIT but can be elevated in both types of AIT. Thyroglobulin levels can be increased in patients with goiters independent of the association with destructive thyroiditis.

In some studies, serum interleukin 6 levels were lower in type 1 AIT and markedly elevated in type 2 AIT. The fact that interleukin 6 is also increased in patients with severe nonthyroidal illnesses limits the specificity of interleukin 6 determination.

Thyroid autoantibodies are generally absent in type 2 AIT. The presence of autoantibodies supports the diagnosis of type 1 AIT. However, a test negative for autoantibodies does not rule out type 1 AIT.

Urinary iodine excretion is not helpful in the initial assessment but may be useful long after the withdrawal of amiodarone to assess whether excess iodine levels are present.

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