Hospital doctors on Teesside have seen the number of new patients drop by nearly 60% during lockdown.
North Tees and Hartlepool NHS Trust saw 6,734 referrals for treatment in April and May, down from 16,100 in the same period last year.
Respiratory consultant Dr Vandana Jeebun said the reduction was across all specialities.
Patients' conditions were worse by the time they did come to hospital, which was a "significant worry", she said.
"We are seeing a trend towards, especially for respiratory recently, an increase in the complexity of the non Covid-related respiratory illnesses," she said.
"It does become harder to manage the patients.
"It's more advanced, so it requires more prolonged treatment, potentially more prolonged hospital stays."
Urgent referrals have continued but some patients have not sought treatment because they are shielding or are worried about exposing themselves to coronavirus.
Others will be waiting until the pandemic is over, thinking "now is not the time to be seeking advice for things which they probably deem less pressing", Dr Jeebun said.
"They need to know that services are still there for those who need it," she said.
The use of digital technology has allowed virtual consultations, improving efficiency and minimising risk to patients.
But more investment in digital innovation and diagnostics capacity is "the way we think we will cope with the rising demand, of not just the backlog, but also potentially more complex patients because of delayed presentation", Dr Jeebun said.