The title of the paper is "The serotonin theory of depression" and it uses the phrase throughout the paper.

Can you tell me where it referenced it as a theory outside of the headline? It also concluded that there is a lack of evidence for the association so why would the authors call it a theory?

Use your browser's search function. At least 4 instances throughout the article

> It also concluded that there is a lack of evidence for the association so why would the authors call it a theory?

Read the article. It explains this in the introduction. Click some of the citations.

"The main areas of serotonin research provide no consistent evidence of there being an association between serotonin and depression, and no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations."

I wouldn't call it a theory especially if you're being semantic about serotonin reuptuke. Perhaps it's you who is using words you don't understand?

The comment you are responding to makes it pretty clear that they know there is not enough evidence for it.

I'm not sure it's worth commenting on the semantic difference between theory and hypothesis, especially since these words often overlap in meaning in casual conversation.

A (scientific) theory is simply a collection of many interlocked observations and logical deductions. A theory can be well established (such as Newton's theory of gravity, or the germ theory of disease), or it can still be at a hypothesis level (such as string theory) or it can be debunked (such as the humor imbalance theory of disease, or the geocentric theory of astronomy).