FGF21 and hypothyroidism: On the contrary, during rapid-onset hypothyroidism following RAI treatment, rising serum FGF21 levels correlated significantly and positively with rising serum triglycerides (Spearman coefficient rs=0.36, P=0.017) and inversely with falling serum SHBG (rs=-0.41, P=0.007) (Figure 4 and Suppl.