Iodination of alkane is carried out in the presence of

$\text{R−H+}{\text{I}}_2\stackrel{\mathrm{h\upsilon }}{\rightleftharpoons }\mathrm{R}-\mathrm{I}+\mathrm{HI}$

HI is a good reductant, it reduces RI to RH 

$\therefore$  Iodination is reversible

To prevent backward reaction, HI must be oxidized.

${\text{HNO}}_3,\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}{\mathrm{HIO}}_3,\hspace{0.17em}\hspace{0.17em}{\mathrm{HIO}}_4$are very good oxidizing agents. They oxidize $\mathrm{HI}$  to ${\mathrm{I}}_2$ and prevents backward reaction.