Today's question is brought to you by my friends with toddlers and young kids. They want to know if thumb-sucking is ok or if it has harmful effects on their kid's speech.
The short answer: it can be.
First of all, let me just say that most thumb-sucking is normal. Babies can even begin sucking their thumbs in the womb. As infants, thumbs and pacifiers can be great soothing mechanisms.
The problem comes in when kids keep sucking their thumbs or pacifiers as their teeth and jaws begin to develop, usually passed the age of 2. Thumb-sucking can actually have the potential of causing oral musculature and formation problems. It can cause incorrect tongue placement and the teeth to form into an open-bite.
When children suck their thumbs, it can affect the position and movements of their tongues, causing it to thrust too far forward or have weak muscle control during speech. The most common sounds that can be affected by incorrect tongue placement are the alveolar sounds: /s, z, t, d, n, l/. Other speech sounds can also be affected. Open bites and tongue thrusts are often a perfect recipe for a frontal lisp.
When a child comes in with an open bite and interdental placement of alveolar sounds, it's usually a long hard road to normalcy during speech therapy. I know some therapists that may not even choose to work with a child who has an open bite. They would prefer to wait until it closes or refer to a dentist.
Overall, if your child is around the age of 2 and still sucking on his thumb or a pacifier, I'd say you might try working to stop it before it negatively affects them.
Here are several other articles and sites that deal with thumb-sucking and speech problems. Be sure and check them out: