I think the feature descriptions for the diabetes set are incorrect. Referring to https://github.com/scikit-learn/scikit-learn/blob/master/sklearn/datasets/descr/diabetes.rst

s1 tc, T-Cells (a type of white blood cells)
s2 ldl, low-density lipoproteins
s3 hdl, high-density lipoproteins
s4 tch, thyroid stimulating hormone
s5 ltg, lamotrigine
s6 glu, blood sugar level

I think tc is probably total cholesterol.

ltg is indicated as "lamotrigine", which I think is a drug for treating epilepsy. My guess is that it stands for triglyceride level.

The Efron et al. paper does not describe the features in any real way, nor where the data came from.