Hypoparathyroidism (HypoPT) is a disease with inadequate production of parathyroid hormone (PTH) from the parathyroid glands leading to hypocalcemia. The most common form is postsurgical HypoPT due to neck surgery resulting in removed or damaged parathyroid glands.

HypoPT is a complex disease with a reduced Quality of life, mild cognitive impairment and in some patients have brain calcifications.

The aim of the present study is to investigate the cognitive function in patients with postsurgical and non-surgical (HypoPT) by neuropsychological assessments and magnetic resonance imaging (MRI).

The investigators will apply a contrast-enhanced MRI based method to HypoPT patients and age- and gender matched controls to examine whether capillary dysfunction can be detected, and whether symptom severity across patients correlates with the degree of capillary dysfunction in certain brain regions. To our knowledge there have been no previous studies on cognitive impairment and its origin in patients with HypoPT. The investigators hypothesize that the symptoms of HypoPT patients represent various degrees of capillary dysfunction, which interfere with their brain function.

Hypoparathyroidism (HypoPT) is a disease with inadequate production of parathyroid hormone (PTH) from the parathyroid glands leading to hypocalcemia. The most common form is postsurgical HypoPT due to neck surgery resulting in removed or damaged parathyroid glands. Postsurgical HypoPT has a prevalence of 22/100,000 inhabitants in Denmark. Nonsurgical HypoPT is most often caused by mutations in different genes or on an autoimmune basis, the prevalence of nonsurgical HypoPT is 2.3/100,000 inhabitants in Denmark. If the gene mutation is within the GNAS gene or upstream of the GNAS complex locus it causes PseudoHypoPT (PHP), which is characterized by target organ resistance to PTH, also resulting in hypocalcaemia and hyperphosphatemia as in post- and nonsurgical HypoPT, but in this case with high or normal levels of plasma PTH. PHP is a very rare disease with a prevalence of 1/100,000 inhabitants in Denmark.

Others and the investigators have previously shown that HypoPT and/or hypocalcemia may affect quality of life (QoL). In most cases, severe symptoms and findings diminished or disappeared when plasma calcium levels are restored to normal, but as some of the above mentioned studies have shown, the reduced QoL is still present despite normal calcium levels. Symptoms are describes as minor cognitive impairment (MCI), with confusion, forgetfulness, and lack of focus and mental clarity. Another important finding in some of the patients with HypoPT is calcifications of the basal ganglia as described in several case reports, the knowledge of the origin and importance of these is so far limited. Patients with PHP have previously been described to have a higher risk of calcifications of the lens and subcutaneous calcifications, and therefore might also be at higher risk of calcifications in the brain.

The oxygen availability in brain tissue is traditionally thought to depend on regional cerebral blood flow (CBF). Accordingly, brain oxygenation is only expected to impair brain function in cases where CBF is critically reduced, cerebral ischemia. This paradigm was recently shown to be in error: While CBF determines the oxygen supply, the microscopic distribution of blood flows across the capillary bed determines the extent to which this oxygen can be extracted by the tissue. In fact, capillary flow patterns can become so disturbed that the metabolic needs of brain function and survival can no longer be met - although CBF remains inconspicuous. Capillary dysfunction denotes this overlooked source of hypoxia: A state in which capillary flow patterns cannot homogenize, giving rise to tissue hypoxia, oxidative stress, local inflammation, and - if severe - long-term neurodegeneration. The investigators hypothesize that the symptoms of HypoPT patients represent various degrees of capillary dysfunction, which interfere with their brain function.

The purpose of the study is to investigate whether HypoPT patients are affected by capillary dysfunction, a condition in which capillary flow disturbances interfere with brain oxygenation.

The hypothesis derives from findings that cerebral calcifications similar to those found in HypoPT are observed in familial idiopathic basal ganglia calcification (IBGC), which is a disease of cerebral pericytes. Pericytes are localized in the basement membrane of capillaries, where they maintain blood brain barrier (BBB) function and - in close interaction with endothelial cells - basement membrane and capillary wall integrity. They are contractile and take part in blood flow regulation. They control the trafficking of immune cells across the vessel wall and possess stem/progenitor cell functions. As such, they play a key role in tissue repair, scarring, and fibrosis, and along with endothelial cells, they seemingly have the potential to become bone-forming cells.

Pericytes (now referred to as 'mural cells') are characterized by several membrane proteins, one of which (SLC9A3R1 - solute carrier family 9, isoform A3, regulatory factor 1, also known as EBP50, NHERF) binds to parathyroid hormone 1 receptor (PTH1R) on one hand, and to plasma derived growth factor (PDGF) receptor β (PDGFβ) on the other, potentiating its activity. The physiological role of these receptors is poorly understood. Receptors for PTH and Parathyroid hormone-related protein (PTH-rP) have been observed in both pericytes and smooth muscle cells of various tissue types, while PTH-rP is expressed in the endothelium, acting as a vasodilator. There is hence reason to believe that the function of capillary pericytes - and hence capillary flow patterns - could be altered in HypoPT.

The investigators have developed a contrast-enhanced, magnetic resonance imaging (MRI) based method to detect disturbances in microvascular flow patterns in the human brain and demonstrated that knowledge of these flow patterns - as indexed by the capillary transit-time heterogeneity (CTH) - allow us to better predict oxygen extraction efficacy as determined by a gold-standard positron emission tomography (PET). The investigators have now found abnormal microvascular flow patterns in patients with Alzheimer's disease (AD) or mild cognitive impairment (MCI) compared to healthy, age-matched controls without cardiovascular risk factors, and demonstrated that the degree of capillary dysfunction correlated with their cognitive impairment. The investigators found a similar correlation in another AD patient cohort, and most recently detected capillary dysfunction in patients with late-onset depression compared to controls. In view of these findings, and the similarity between the symptomatology of HypoPT and depression, the investigators now propose to apply our MRI method to HypoPT patients and age- and gender matched controls to examine whether capillary dysfunction can be detected, and whether symptom severity across patients correlates with the degree of capillary dysfunction in certain brain regions.

The aim is to investigate the cognitive function in patients with postsurgical, non-surgical and pseudoHypoPT by neuropsychological assessments and MRI. The contrast-enhanced MRI based method will be applied to HypoPT patients and age- and gender matched controls to examine whether capillary dysfunction can be detected, and whether symptom severity across patients correlates with the degree of capillary dysfunction in certain brain regions.

The investigators hypothesize that the symptoms of HypoPT patients represent various degrees of capillary dysfunction, which interfere with their brain function.

Methods:

MRI scans: The investigators plan to acquire the following MRI sequences:

1. Dynamic susceptibility contrast perfusion MRI will be applied for measurement of capillary function and regional blood flow and volume as estimated by our by parametric approach.
2. For further characterization of the microcirculation, resting state functional MRI will be acquired. This sequence can be acquired in combination with the perfusion scan and thus requires very little extra scanning time.
3. Fluid attenuated inversion recovery (FLAIR) images will be acquired for estimation of white matter hyperintensities (WMH) and possible subcortical infarcts or lacunas.
4. A fast diffusion kurtosis imaging (DKI) sequence will be applied for estimation of microstructural integrity of gray and white matter (e.g. dendrite density and white matter tract integrity).
5. T1-weighted MRI will be utilized for accurate morphological characterization of cerebral structures, such as hippocampus, basal ganglia, cortex and major tracts. In addition, the high-resolution images enable accurate mapping of the functional data (perfusion, fMRI, DKI).

Neuropsychological testing (NPT): The standard neuropsychological test battery consists of validated tests to assess cognitive domains including verbal and visual memory, attention, language, visuospatial and executive functioning.

Cognitive batteries: Cogstate battery for assessing cognitive function in adults with mild cognitive impairment (MCI). The tests used are D-KEFS: TrailMaking A and B, Verbal Fluency Test, and Color-Word Interference Test. The WAIS-IV: Coding, Symbol search, Digit span, and Arithmetic. The BVMT-R: delayed recall and total learning. Lastly the RAVLT: delayed recall and total learning.

Questionnaires: SF36v2, WHO-5 well-being index, Symptom questionnaire, and a general questionnaire on background information, medication and diet, and HPQ28.

Biochemistry: Ionized calcium, PTH, phosphate, magnesium, eGFR, creatinine, High sensitive CRP, 25-hydroxyvitamin D, calcitriol, and white blood cell count. Neuroinflammation and systemic inflammatory markers (pro-inflammatory cytokines), and 24-hr urine.

Statistical Analysis Plan: Group difference will be examined using student T test for continuous variables and Pearsons X^2 test for categorical variables. Statistical maps of differences in perfusion parameters between patients and controls will be calculated at each surface vertex using a vertex specific general linear model with cortical thickness, white matter hyperintensity load ((WMHL) given as percentage white matter hyperintensities volume of whole brain volume)), age, and gender as covariates. The investigators will add cortical thickness as covariate to account for any systematic effects of cortical thickness on the perfusion measurements, such as partial volume effects. All statistical maps will be thresholded at p=0.05 (uncorrected and corrected). All statistical tests for the MRI scans are carried out using R version 3.2.2 (The R Foundation for Statistical Computing) and SPM12 (Wellcome Trust Centre for Neuroimaging) running on Matlab R2016a (MathWorks Inc). Other statistical test will be carried out by SPSS 24 (IBM, USA) Justification for sample size: In a previous study by Eskildsen et al. the contrast-enhanced MRI based method has been applied to 18 patients with Alzheimer's disease or MCI compared to 19 healthy, age-matched controls without cardiovascular risk factors and it demonstrated that the degree of capillary dysfunction correlated with their cognitive impairment. The investigators found a similar correlation in another patient cohort with Alzheimer's disease, and most recently detected capillary dysfunction in patients with late-onset depression compared to controls. Taking the above mentioned results and the sizes of the cohorts in consideration, the investigators have estimated that our sample size is enough to show if there is a correlation between cognitive impairment measured by neuropsychological assessment and capillary transit time heterogeneity.

Safety and ethnical evaluations: The investigators will follow the standard operating procedures regarding contrast enhanced MRI scans. Peripheral venous catheter will be used for administration of PET tracers and MRI contrast. It can result in mild local pain and a hematoma. The risk of infection is negligible. There are no known risks (neither short- nor long-term) related to MRI. Gadolinium-chelate (Gd) contrast is used. The standard dose of gadolinium (Gd)-containing MRI-contrast is 0.1 mmol/kg, but in this study there will be used up to 0.3 mmol/kg.

Perspectives: If successful and study findings provides a basis for improved understanding of cognitive symptoms in HypoPT, further investigations on effects of PTH replacement therapy on investigated indices should be considered. A future project could be performing the MRI scan before and after injection of PTH to determine whether a change in capillary transit time heterogeneity could be detected.

Inclusion Criteria:

• Patients:
• Male or female with age between 18 and 70 years.
• A low endogenous PTH production as verified by low plasma levels of intact PTH, necessitating treatment with 1-αhydroxylated vitamin D analogs.
• HypoPT for 3 years with continuous alphacalcidol or calcitriol treatment prior to study entry (except for the patients with PHP).
• Stable P-calcium levels 1 month prior to inclusion.
• In case of thyroid disease, TSH within reference range within the last year
• Speaks and reads Danish

Controls:

• Male or female with age between 18 and 80 years.
• No known diseases in the calcium homeostasis
• Speaks and reads Danish

Exclusion Criteria:

• Reduced kidney function (eGFR < 30 mL/min/1.73m2).
• Diabetes type 1 or 2
• History of hypertension for more than two years (treated or untreated)
• Clinical suspicion of major depression (also if treated)
• Clinical suspicion of alcohol-related dementia
• Other organic or psychiatric cause the patients symptoms
• Contraindications to contrast-enhanced MRI.
• Metal implants close to the head, which will interfere with the MRI or pacemaker. The patients will complete a metal scheme.
• Claustrophobia
• Unwillingness to participate

The patients with postsurgical, non-surgical and pseudo hypoparathyroidism will be invited from our outpatient clinic to participate in the study.

The healthy controls will be randomly recruited from the background population by an extract from the CPR register and an invitation to participate in the study will be send by electronic letter.