Diagnosing hypothyroidism?

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I was wondering if anyone can advise me about how this is diagnosed in children? DS (5) has a heap of health worries. I've posted in detail before about his bladder/ bowel issues (both appear to be underactive, resulting in leaking urine & constipation/ rascal overflow). He also has eczema, asthma and is prone to chest infections/ croup which is currently being treated as asthma.

Now his nails have started splitting vertically; one of them half of the nail came off yesterday. It rang a bell that nail issues can signify underlying health issues and apparently hypothyroidism can be a cause of this. So that's nearly all of his health worries that would be accounted for by one condition.

I'm wondering whether I need to ring the paediatrician to discuss this? He had a lot of blood tests in Jan and I know "thyroid" was one of them and was apparently normal, but I have read on here and elsewhere of people not being diagnosed with the first tests that they have by a different test, so I was wondering whether it is worth revisiting? If so what are the tests I should push for?

It sounds like a worrying time. For thyroid tests, it's useful to see the results to check if they are borderline. Sometimes only tsh is tested but really you need ti test ft4 (and ft3 but that's really tested in nhs) and antibodies for autoimmune thyroid disease. Hopefully the usual haa been tested - vit d, iron, folate, vit b12.

Thanks @Imagine123 on the one hand he's pretty cheerful and not in any great pain so I doubt myself when I'm thinking of being pushy. On the flipside, I'm terrified of dropping the ball for him, poor lovely. I'll ask about whether the tests were borderline.