The brief letter by Michael White  talking about how: "Clinicians need to be mindful of the possibility of undiagnosed autism in all patients with TRD [treatment-resistant depression]" was a bit of an eye-opener. White who lists "has an adult son with autism" on the 'competing interests' section of his publication, offers an interesting perspective in relation to TRD (described as those "who do not respond adequately to a course of appropriate antidepressant medication within a certain time").
The letter from White is brief but covers quite a few important issues including (a) the observation that autism rarely exists in a diagnostic vacuum when it comes to psychiatric and/or behavioural labels (see here) and (b) the quandary of whether depression occurring alongside autism is a 'comorbidity' or something rather more 'core' to at least some autism (see here). That last point in particular is an important one given how many times depression has cropped up in the peer-reviewed research literature with autism in mind and onward whether certain typically indicated interventions for depression are actually cutting the clinical mustard where autism is mentioned (see here).
I am slightly cautious not to go 'all in' with the idea that every case of TRD is just undiagnosed autism. There are multiple other potentially important things to consider where depression - treatment resistant or not - presents (see here and see here for examples) over and above a diagnosis of autism or the presentation of clinically significant autistic features. But there could be some merit in physicians at least thinking about undiagnosed autism as and when TRD presents...
 White MJ. Treatment-resistant depression: consider autism. Br J Gen Pract. 2019 Jan;69(678):14.