If TSH levels are above the reference range, the next step is measurement of T4. T4 is highly protein bound (99.97%), with approximately 85% bound to thyroid-binding globulin (TBG), approximately 10% bound to transthyretin, and the remainder bound loosely to albumin.
The levels of these binding proteins can vary by hormonal status, inheritance factors, and in various disease states. Hence, free T4 assays, which measure unbound (ie, free) hormone levels, are becoming popular. However, a free T4 assay can be unreliable in the setting of severe illness or pregnancy. Free T4 level can be directly measured via equilibrium dialysis. Results are independent of binding protein concentrations. However, this test is more costly and generally takes longer to return.
The TRH stimulation test is an older and rarely needed test for helping to assess pituitary and hypothalamic dysfunction. With the improvements in assays to measure TSH and free T4 levels, TRH stimulation testing has become outmoded. In the United States, this test is available only at the National Institutes of Health.
Assays for anti-TPO and anti-Tg antibody levels may be helpful in determining the etiology of hypothyroidism or in predicting future hypothyroidism. However, once a patient has been found to be antibody positive, repeated antibody testing adds little to the clinical picture and thus is not recommended.
The complete blood count and metabolic profile results may show abnormalities in patients with hypothyroidism. These include anemia, dilutional hyponatremia, hyperlipidemia, and reversible increases in serum creatinine levels. Elevations in the levels of transaminases and creatinine kinase have also been found.
Read more on the workup of hypothyroidism.
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Cite this: Romesh Khardori. Fast Five Quiz: Key Aspects of Hypothyroidism - Medscape - Apr 15, 2019.