FGF21 and hypothyroidism: However, there was a significant transient increase in FGF21 serum levels in rapid-onset hypothyroidism in comparison to hyperthyroid and euthyroid phase [median 160.55 (interquartile range, 92.48-259.35) vs 119.55 (67.78-192.32) and 104.43 (55.93-231.93) pg/mL, P=0.034 and 0.033, respectively] (Table 2 and Figure 3).