The hospital trip came as concern over her condition has intensified: Haakon said in May she had worsened, and King Harald recently said she was seriously ill.
Mette-Marit, diagnosed with chronic pulmonary fibrosis in 2018, has appeared publicly with supplemental oxygen, and the palace said last winter doctors were preparing for a possible lung-transplant evaluation.
The strain has already disrupted royal plans: Princess Ingrid Alexandra returned from college in Australia to be with her mother, and Haakon cut short an official trip to Japan.