Ever hear "Go to the source"?
Well, for medical coders, the source is CMS. This where where US medical coding information originates.
For a free introductory course in the ICD-10, you cannot beat CMS. They have online instruction covering both CM, and PCS.
If you need a review or are new to medical coding, these sites are your springboard.
The site has other courses on their main page, and I highly recommend taking advantage of all learning opportunities. Always :)
For those of you thinking "That's wrong! WHO is where coding begins!", you are not wrong. You are also not correct. The World Health Organization is the birthplace of the ICDs. However, the ICD CM/PCS is specifically an American code-set. All the WHO countries can (and do) modify the uber-code-set to suit their unique needs. So for readers seeking information on coding in the US, CMS is the wellspring.
WHO runs a learning site called openwho.org, a great site for learning but they do not currently have medical coding specific info. They have nothing specific to coding in any particular country. Which is why, If you go there, don't panic at all the ICD 11 information. While it has been implemented is some countries since 2022, the US is so far from switching to 11 there isn't even a published target date (at time of writing).
A few bonus course ideas are given in this nurses.org article. Sadly OpenLearn offers nothing specifically coding for medical coders, but does have general topics related to. Udemy is seldom free, however it has a lot of medical coding courses (over 7K of them!) for very low cost. AMCI sometimes offers a free beginning course (their free YouTube videos got me through my CPC), and there are many other good YT vids. Quizlet is another not-to-be-missed site. Finally, Noridian webinars are very helpful.