Jack of all trades. Spam item actives.
Durability and crowd control
After immobilizing an enemy champion gain defenses and later deal a burst of adaptive damage around you.
Impairing the movement of an enemy champion marks them. Your allies heal when attacking champion's you've marked.
Gain 5 Armor. Whenever you heal for at least 20 health or from a consumable's effect, increase your Armor by 5% for 3 seconds.
Increase your Tenacity and Slow Resistance for each summoner spell on cooldown. Immediately after casting a Summoner Spell, gain a larger burst of Tenacity and Slow Resistance for a moderate duration.
+65 health and +9 AP or +5.4 AD, adaptive
Deals bonus true damage to enemies with impaired movement or actions.
After killing an enemy ward, a friendly Zombie Ward is raised in its place.
Like Courage but with damage, Thresh can easily proc Aftershock and it adds to his kill potential if you can land the explosion.
Font of Life is a great rune on Thresh as he has multiple tools of proccing it with [Q], [E], and [R], providing a lot of healing in lane trades and team fights.
The Armor from Iron Skin is incredibly valuable in lane trades, especially when looking for that level 2 engage.
Unflinching allows you to stick to your target for repeated CC and disruption after flashing in for the engage.
Like Font of Life, Thresh has 3 ([Q], [E], and [R]) strong tools for proccing Cheap Shot empowering your lane for even more kill pressure.
To key to being a useful support is effective warding and Zombie Ward shines in this department.