I'm in the same boat, last time I checked. Outwardly, I am very healthy. Lean young dude. The first time I had my cholesterol checked, I think it was around 250, maybe a little lower. A few months later, up to 270. A year or so later at a health screening, over 300. Went to my real doctor and got NMR and LDL-P was over 2000, so that made me quite concerned.
The past few months, I've upped my carb intake in general and also increased my consumption of seaweed (for iodine) and a Brazil nut daily for selenium. All these things are thought to support thyroid function, specifically the conversion of T4 to T3 (the active form). In my case, this is just a theory. I asked for a "full thyroid panel" and got T4 and TSH, which tells me nothing. SO if you do get your thyroid checked be SURE to ask for T3 and rT3 in addition to TSH and T4.
Hypothyroidism | Perfect Health Diet
Iodine and selenium | Perfect Health Diet
Another theory I've seen is that certain APOe variant carriers are more susceptible to elevated cholesterol on a high fat diet. Lauric acid (found in large quantities in coconut oil) in particular is good at raising LDL in some people.
Effects of dietary fatty acids and carbohydrates on the ratio of serum total to HDL cholesterol and on serum lipids and apolipoproteins: a meta-analysis of 60 controlled trials
How can I safely and quickly (within 2 months) lower my total cholesterol? - PaleoHacks.com
Also, the posts on the blog here recently by Dr. Peter Attia are great. His 9 part series on his blog goes into even greater detail. If you read the series, you will almost certainly know more about cholesterol than your doctor.
The Straight Dope on Cholesterol: 10 Things You Need to Know – Part 1 | Mark's Daily Apple
The Straight Dope on Cholesterol: 10 Things You Need to Know – Part 2 | Mark's Daily Apple
The straight dope on cholesterol