https://www.endotext.org is a good website for endocrinology.
In-depth Textbooks typically Williams or Greenspan's, not sure about Harrison's but probably all viable. I don't know how Khan academy is for an intro but possible. Maybe Guyton medical physiology for general physiology and general coverage for endocrinology.
Perhaps there are some additives/binders/etc... that can change across generics and some brand-name preparations. Could be possible absorption issues, as the thyroid medications can be finicky (avoiding PPIs, cal/mag, etc...). As for source (pork thyroid extract vs. Levothyroxine (synthroid and all)) there could be areas to speculate on but nothing definitive. T3 vs. T4 certainly different.
As for your other comment "in their mind", there can be a good deal of neuro hormonal overlap and anxiety / other disorders are common. However, anecdotally, a tiny/ small subset of people with 'thyroid disorders' can feel .2 difference in some TSH (not accounting for lab range) and other values, are allergic to psychiatric treatment/therapy (everything is due to my thyroid), allergic to basically every treatment (except maybe armour thyroid and vicodin [pre-2014]), refuse everything not 'natural', can be quite somatic, and sometimes suddenly change their tune when having to pay a full price for brand vs. a covered generic. I remember from an endocrinology clinic I helped support (electronic records), one of the docs remarked on that tiny subset for taking up her time from more endocrinologically actionable cases. I think she was a little harsh (though very caring in person), but there you go. Also calling the pharmacies all the time and complaints.
Edit: felt like the last paragraph was a little too small in scope/ answering a specific case relative to the question. Of course when you look at entities like NMDA encephalitis and untreated hyperthyroidism or hypothyroidism you can see clear connections. Mood swings on Hashimoto's to the point of getting the thyroid ablated, etc... Autoimmune reactions more common, allergy/hive/immune axis.
Must be mentioned that levothyroxine is the #1 or near the top by prescriptions (over 100 million , so you will also have a large pool of people to work from and who could report symptoms or find others). I would be curious to see how the percentage of people intolerant to some thyroid medication are also intolerant to forms of other medications relative to those not. Perhaps the Brand-name only crowd or highly intolerant to x could be weighted more heavily. Could be individual interactions. Lots going on.
