Hair thinner towards the root although I have a full head of hair? (Photo)

This is due to miniaturization. I suggest seeing a hair loss expert now to evaluate and go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

Hello Privatepatient25,
Really not enough information to make a judgement call. Recommend having a hair evaluation performed. If a problem appears to be developing it is likely a PRP treatment or two followed by a maintenance treatment every 6 months will nip it in the bud. 
There are many PRP hair regeneration protocols today. So many so that many physicians are actually confused. If indeed you elect to try one, make sure it makes sense.
The greater the platelet density in combination with correct concentration in the treatment area are the biggest keys. Many of the physicians who answer questions on Realself, including myself have posted protocols in our prior answers. I recommend you research them.
I also recommend you have your hormones checked by a good Integrative Medicine physician (IMP). If for example, your DHT is getting high, bringing it down will most definitely help. They will also be more likely able to treat it more naturally than first going to something like Propecia that carries undesirable side effects for many.  
IMP's are not against Propecia per se. It and all other medications remain in our toolbox. We simply prefer to first try other more natural or gentle alternatives first.
Hair is complex, but as you suspect, it sometimes can be an indicator of other issues. An IMP will identify if that is the case or not. Don't worry. More than likely it is something like DHT causing the issue. If so, its generally easy to correct and far cheaper than chasing most of the remedies on the market.
Don't wait, earlier identified problems are much easier to correct.
I truly hope you are able to identify the issue, are easily able to treat it and enjoy a lifelong full head of hair.
Feel free to ask more questions, we're here to help you thrive.
Warmly,
Arbella Sarkis, MD       

This could be completely normal depending on how many such hairs you have. If there are a lot, this suggest a dystrophic hair and potential inflammation at the level of the bulb or acute stress on the follicle. Miniaturization would appear different as it is just thin throughout. A hair that is thick at the top and thinner at the bottom has disrupted growth - this is completely different.  A hair collection can nicely answer your question as to whether this is normal or not. 

When hair is miniaturiazing,the newer growth of the hair shaft becomes thinner and when it falls outm you see this change as you observed