I have a rash on my hands. the skin keeps splitting on my thumbs.

Rash: Commonly eczema, due to dry skin, specifically with seasonal variation ( worse during winter) and an over the counter moisturizer or low strength steroid ointment may suffice. Not as common occupational/ infectious or reflecting other medical condition, such as hypothyroidism, diabetes or rheumatologic disorder, among others. Hope it helps.

Rash Hands: It could be contact dermatitis or thrush .Need to confirmed by examination. Please see doctor or post question with picture.

Irritant dermatitis?: Hard to be sure from your brief description but one of the very common causes for this problem is irritant contact dermatitis where the skin of your hands is repeatedly exposed to something like detergent or water or a chemical (often work related). Have a look at this excellent website to see if your symptoms fit: https://www.dermnetnz.org/topics/irritant-contact-dermatitis/

Contact dermatitis : It could be contact dermatitis ( allergy to some chemical ) or thrush ( fungal infection) . Would be worthwhile seeing doctor for examination.

Need to be seen: Likely you have irritant dermatitis as the others say and this will likely respond to eczema type treatments and avoidance of irritants. There are some other skin conditions including fungal so as the others have said needs either assessment or virtual consult with images (text with photos or video consult). All the best.