A t-shirt that has gone translucent across the back is not worn out in any structural sense. The seams are intact, the collar may still be fine, nothing is torn. It is simply unwearable, which is a worse kind of failure because there is nothing to point at.
This is predictable, and largely avoidable. Here is what is actually happening.
Cotton sheds, constantly
Jersey knit is made from spun yarn — short fibres twisted together into a continuous thread. Every wash and every wear works some of those fibres loose. That is what lint is. The fluff in your dryer filter is your clothes, leaving.
As fibres escape, the yarn gets thinner and the gaps between the loops of the knit get wider. Light passes through those gaps. The fabric has not stretched; it has thinned.
Every cotton garment does this. What varies is how much material there was to begin with, and how fast it leaves.
Three things decide how fast it happens
Starting weight. This is the big one. A 150 gsm tee that loses 15% of its density is visibly sheer. A 260 gsm tee that loses the same 15% is still opaque, because it started with far more fibre per square metre. Weight buys you time.
Fibre length. Long staple cotton — Supima, Egyptian, and similar — has fibres several centimetres long, so each one is anchored into the yarn along more of its length and works loose more slowly. Short staple cotton sheds and pills faster at any weight. This is why a well-made midweight tee can outlast a badly-made heavyweight one, and it is also why fibre quality rarely appears on a label: it is expensive and hard to verify.
Spinning method. Ring-spun yarn is twisted tighter and more evenly than open-end yarn, which holds the fibres in place better and gives a smoother surface. Combed and ring-spun is the phrase to look for. Its absence usually means open-end.
What you are doing that accelerates it
Three habits do most of the damage, and all three are easy to change.
Tumble drying on high. This is by far the worst thing you can do to a cotton tee. Heat and mechanical agitation together tear fibres loose faster than anything else in the garment's life. Dry low, or hang dry and accept the extra hours.
Washing hot. Hot water swells and weakens cotton fibres, and it sets the shrinkage. Cold at 30°C cleans normal sweat and wear perfectly well. Hot water is for stains and illness, not for a shirt you wore to the gym.
Overloading the drum. A packed machine means garments abrade against each other for the whole cycle instead of moving through water. More friction, more shedding. Wash inside out, and give the load room.
Where it goes first, and why
Thinning is never even. It shows up first across the upper back and the shoulders, because that is where the fabric is held under the most tension against the body and where a backpack, a bench or a bar makes contact. The lower front stays opaque longest.
If you want to check a shirt honestly, hold it up to a window rather than looking at it on a hanger. Daylight through the upper back panel will tell you where it is in its life.
The cost argument
A cheap tee is not cheap if you replace it three times. A shirt at a third of the price that lasts a quarter as long has cost you more, and it has spent most of that time looking worse.
This is the whole reason we build at 260 gsm. Heavier than a tee strictly needs to be, because a shirt that goes translucent after ten washes was not a bargain — it was just gone sooner. The extra fibre is not there for feel. It is there so that the inevitable shedding takes years instead of months.
None of which makes cotton permanent. It shortens the odds.