If TSH levels are above the reference range, the next step is to measure free T4. Another option is to measure total T4 and binding proteins. The levels of these binding proteins can vary by hormonal status, inheritance, and in various disease states. Hence, free T4 assays, which measure unbound (ie, free) hormone, are the accepted standard.
Free T4 assays can be unreliable in the setting of severe illness or pregnancy. In pregnancy, the variation in the results of commercially available free T4 assays has led the American Thyroid Association to recommend using method-specific and trimester-specific reference ranges for serum free T4. If these specific ranges are not available, TSH, total T4, and free T4 index can be used to monitor the pregnant patient.
Patients with primary hypothyroidism have elevated TSH levels and decreased free hormone levels. Patients with elevated TSH levels (usually 4.5-10 mIU/L) but normal free hormone levels or estimates are considered to have mild or subclinical hypothyroidism.
Primary hypothyroidism is virtually the only disease that is characterized by sustained rises in TSH levels. As the TSH level increases early in the disease, conversion of T4 to triiodothyronine (T3) increases, maintaining T3 levels. In early hypothyroidism, TSH levels are elevated, T4 levels are normal to low, and T3 levels are normal. Given this early protection of the T3 level, routine checking of T3 is not recommended if one suspects that a patient is hypothyroid. Drawing a reverse T3 is also not recommended as a routine part of the hypothyroidism workup.
Learn more about hypothyroidism workup.
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Cite this: Romesh Khardori. Fast Five Quiz: Hypothyroidism Practice Essentials - Medscape - Apr 11, 2022.