Thick, yellow nails that grow slowly often point to lung diseases like emphysema or chronic bronchitis. Fluid in your lungs and hands also can lead to yellow nails. So can Raynaud’s phenomenon, which is caused by poor circulation to the fingers, toes, and nose. Yellow nails can even be a symptom of sinusitis, thyroid problems, and rheumatoid arthritis. Only your doctor can tell you what they mean.