*5.2.1 Thyroid hormones*

Serum TSH measurement has the highest sensitivity and specificity of any single blood test used in the evaluation of suspected thyrotoxicosis [44]. Diagnostic accuracy improves when a serum TSH, free T4, and total T3 are assessed at the initial evaluation. In overt hyperthyroidism, serum free T4, T3, or both are elevated, and serum TSH is subnormal (usually <0.01 mU/L in a third-generation assay). In mild hyperthyroidism, serum T4 and free T4 can be normal, only serum T3 may be elevated, and serum TSH will be low or undetectable. These laboratory findings have been called "T3-toxicosis" and may represent the earliest stages of hyperthyroidism caused by GD [45]. Ratio of total T3 to T4 may also be useful in differentiating GD from thyroiditis. In one study the ratio of total T3 to total T4 (ng/lg) was >20 in GD and toxic nodular goiter, and <20 in painless or postpartum thyroiditis [46].
