A thyroid hormone called T3 turns up a gene called SF-1 in mouse testicle cells, and without SF-1, T3 can’t make the cells produce sex hormones—so SF-1 is like a middleman helping the hormone do its job.

When there’s too much of a protein called DAX-1 in mouse testicle cells, it blocks the thyroid hormone from doing its job—preventing the cells from making progesterone, a hormone needed for reproduction.

Two different hormones, T3 and hCG, both make a key protein (StAR) in mouse testicle cells work harder to produce steroids, and when you use both together, they boost it even more — like two different keys that each unlock the same door, and using both at once opens it wider.