Public Defence: Stephen Hewitt Stephen Ingemar Hewitt at Institute of Clinical Medicine will be defending the thesis “Secondary hyperparathyroidism after obesity surgery - exploring the impact of vitamin D and calcium, and aspects on bone health” for the degree of PhD (Philosophiae Doctor).

Due to copyright reasons, an electronic copy of the thesis must be ordered from the faculty. In order for the faculty to have time to process the order, it must be received by the faculty no later than 2 days prior to the public defence. Orders received later than 2 days before the defence will not be processed. Inquiries regarding the thesis after the public defence must be addressed to the candidate.

Trial Lecture – time and place

See Trial Lecture.

Adjudication committee

  • First opponent: Professor II Bård Kulseng, Norwegian University of Science and Technology (NTNU)
  • Second opponent: Senior Consultant Eva Szabo, Örebro University Hospital, Sweden
  • Third member and chair of the evaluation committee: Professor Yvonne Böttcher, University of Oslo

Chair of the Defence

Professor Theresa Mariero Olasveengen, University of Oslo

Principal Supervisor

Professor II Tom Mala, University of Oslo


Recent studies have shown bone loss and more fractures after obesity surgery. Blood levels of parathyroid hormone (PTH) are frequently increased in these patients, while the effect on bone has been questioned. Sufficient vitamin D levels can counteract PTH elevations and have benefits on bone health.

Stephen Hewitt and co-authors investigated the prevalence of PTH elevations and relationships with vitamin D and calcium after obesity surgery in the PhD thesis.

The thesis is based on four studies after the gastric bypass operation, a commonly performed procedure for morbid obesity. All the 4 studies found a high prevalence of PTH elevations (29-40%).

In two of the studies, the PTH disturbances decreased with vitamin D levels of at least 50 nmol/l, in accordance with traditional recommendations, whereas levels from around 100 nmol/l showed additional improvements by 40-50%.

A relationship between the blood levels of calcium and PTH was shown, and the calcium balance was improved with higher vitamin D levels.

The thesis found a relationship for the PTH elevations with higher bone turnover and with lower bone mass. Patients presenting with osteoporosis at 10 years, had undergone around three times more low-energy fractures, compared with patients showing a more normal bone mass.

The thesis has described a relationship between the PTH elevations and impaired bone health after obesity surgery. Vitamin D levels of at least 50 nmol/l reduced the PTH disturbances and improved bone metabolism. Vitamin D levels from around 100 nmol/l and higher may reduce the PTH disturbances further. The thesis may help to improve our understanding of the relationship between vitamin D, calcium, PTH and bone health after gastric bypass.

Additional information

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Published Feb. 3, 2022 10:34 AM - Last modified Feb. 17, 2022 12:34 PM