Brief Answer:
teething irritation is the probable cause...

Detailed Answer:
Hi there,
Welcome to the HCM,
This is because of eruption of teeth leading to the irritation of gums. Babies do put fingers in their mouth to satisfy their this irritation of gums,

Later on this becomes a habit . Many times babies do become addicted to this habit.
To control this habit is very difficult.

Diverting the mind of the baby with some interesting activity will be of some help. like showing some cartoon channel in TV.

Putting some noxious taste fluid etc in the fingers especially in the nights.
This is also a very important cause of infection in the baby especially stomach infection(frequent loose motions).

Yes appetite is also reduced as when ever baby is hungry he put his finger in his mouth to fulfil his appetite.

Hope the query is answered.

Please feel free for the further clarification.
thanks

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