I have been main-ing support thresh bottom, usually as the only tank on the team as well. So i have gotten my items down very well for the survivability i need against most teams and wanted to share it.

The gold per ten and armor runes i run are to help with the early game lane fights that i initiate as thresh and to make sure i can keep up in items as i tend not to build gold per ten items or farm, relying on assists for my main source of money.

8 of the .25g/10 seals gives 2 gold/10.
3 of the 1g/10 quintessences gives 3 gold/10.
= total of 5g/10.

1 of the 1.41 armor seal.
9 if the .91 armor marks.
9 of the .7 armor glyphs.
= total of 15.9 armor.

I go 0/18/12 for the extra defense but still able to get some extra gold per ten to help keep me in the running to get some good items on time.

In the defense tree the main talents (in my opinion) are the reduced tower damage, reduced slow amount, reduced cc time, and increased max health.

In the support tree the main talents are the gold per ten, mana regeneration and summoner spell cdr.

This way you can remain in the fight longer and not be as harshly hindered by enemies and spam your summoner spells almost every fight while still being able to get your items quicker and not running out of mana.

Start out with 3 wards 2 potions (health, mana, or one of each) and rejuvenation beads. Rush your sightstone so you don't have to waste money on wards, then get boots 1 and aegis of the legion. From there upgrade your boots, then either upgrade your aegis (if the enemy team does a lot of magic damage) or get warmog's armor.

From there you get choices.
I prefer soul shroud and sunfire cape for the extra health, health regen and resistances as i am usually main tank. Guardian Angel is also a good item in this space, which i would replace sunfire cape for since you can farm armor with your passive.

If you are not main tank, and want to focus in damage you can go for rylai's crystal scepter or abyssal scepter, or possibly (but i dont care for it) rabbadon's deathcap

And lastly, if you are not main tank and want to focus support you can go for zeke's herald or will of the ancients to increase your team's damage OR locket of the iron solari and mikael's crucible for extra team durability in those team fights.

I tend to go Flash Exhaust.

Flash can help you set up plays or escape. With the 10% summoner spell crd and the 15 seconds lower cd from the support tree you can use it a lot.
Exhaust helps protect your allies or helps secure a kill, a great tool for any support.

Other spells:
I like to see heal, clairvoyance or teleport on thresh.

Heal helps keep your team alive as a support should do.
Clairvoyance gives you that extra eye in the sky, preventing ganks and barron attempts all game long.
Teleport allows for quick movement around the battlefield, never being too far from that team fight breaking out when you are needed.

So, in my guide (if you couldn't tell already) thresh is supposed to be there to soak up the damage and nab those kills for your allies. I focus tankyness as i am usually the only tank on the team, and even if i am not i have to be alive to keep the enemies off of my carries as thresh can do so very well. In the end i get whatever the team needs at the time, but like to go for the extra health and resistances and tend to feed our adc during the laning phase.