Which lab tests are best when you suspect hypothyroidism?
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Thyroid-stimulating hormone (TSH) level is the preferred test for initial evaluation of suspected primary hypothyroidism (strength of recommendation [SOR]: C, expert opinion). If TSH is abnormal, a free thyroxine (T4) level will further narrow the diagnosis. Obtain a triiodothyronine (T3) level if TSH is undetectable and free T4 is normal. When assessing the adequacy of replacement therapy in primary hypothyroidism, the TSH is the most important parameter to monitor (SOR: C, expert opinion). Because TSH levels can't be used to monitor central hypothyroidism, use free T4 and T3 concentrations (SOR: C, case series).
Journal of Family Practice, 57(9) 2008: 613-614.