Can you start using the NHS as soon as you enter with a spouse visa?

So I went to a final follow up appointment with my US doctor today, and there was an unexpected medical problem discovered from some past lab results. I'm going to try to get in to see a US doctor, but I have less than 2 weeks left so I doubt I can get in. Can I use the NHS right away or do I have to wait for my NI number to come in the mail? I would most likely go straight away to a Reproductive Healthcare clinic if that makes a difference.

Yes, you can.

Your NI number has absolutely nothing to do with getting NHS treatment - it is used to work out how much tax you pay on your income... it's like a SSN. An NI number is only issued to UK citizens aged 16 or over who have reached working age, or to people who have moved to the UK and wish to work.

When you register with an NHS GP, you will be automatically issued with an NHS number (this is usually issued to UK citizens at birth/when they have their first medical appointment as a baby)...which is a unique number in the NHS system, used to track your medical history. However, you will need proof of your UK address (and your passport and BRP) in order to register, so you'll likely have to wait until you have some mail in your name.

If you need treatment quickly, you could probably either register as a temporary patient until you have proof of address, or you might be able to get treatment at a Family Planning Clinic, but it may depend what kind of treatment you need.  You can also use the A&E for emergency treatment if you have to.

And if it isn't an emergency situation but you need to see someone urgently, call 111, which is the NHS non emergency help line.  They give medical advice but can also make appointments and send ambulances as needed.  I have used it many times (I work in a non medical setting but have had to call ambulances and perform first aid for staff or customers a number of times) and they are excellent.  If you are not yet registered to a GP they can make you an urgent care or walk in clinic appointment.  I also suspect a few GP practices hold back a handful of appointments for 111 bookings as they seem to be able to override the whole 'We're fully booked today, call back tomorrow at 8' thing.

Thank you guys for your quick reply! Thankfully I was able to get in with a doctor yesterday(I couldn't believe they got me in the same day) and everything turned out ok! So I shouldn't need to use the NHS right away but I'm glad that if something happens that I can get treated right away.

Great news - well done.

Excellent!!!