Longitudinal Epidemiologic Studies of Cancer and IIH Risk Following Long-Term Gender-Affirming Hormone Therapy

What the evidence says

The evidence on vitamin D status and cancer risk splits along methodology. A body of observational research links low serum 25(OH)D concentration to increased risk across multiple cancer types — colon, breast, ovary, prostate, pancreas, skin, kidney, brain tumors, multiple myeloma, and leukemia — with high confidence in the direction of the association but low confidence in its magnitude, since the claim does not report effect sizes. Randomized controlled trials of vitamin D supplementation, by contrast, have not reproduced a consistent effect on cancer or other health outcomes; the source attributes the inconsistency to short follow-up periods, differing supplementation regimens, and varying doses across trials. A separate, single cross-sectional study finds that greater income inequality is associated with higher cancer mortality among people aged 65 and older, alongside lower cardiovascular mortality and no measured effect on infectious disease mortality in the same age group; this rests on one study with limited methodological detail and unclear control for individual-level income, so both direction and magnitude confidence are limited.

On the genetic side, the evidence establishes causal mechanisms rather than population-level risk estimates. CYP24A1 gene mutations are established to cause autosomal recessive idiopathic infantile hypercalcemia (IIH) through loss of function of the enzyme 24-hydroxylase. That same enzymatic loss of function does not always present at birth: mutations described in the literature have manifested clinically as late as adulthood despite being present from birth, meaning the same genetic lesion can produce a delayed clinical picture rather than a fixed one. A distinct gene, SLC34A1, produces a related but mechanistically separate outcome: mutations in this gene, which codes the renal sodium-phosphate co-transporter NaPi-IIA, cause excessive synthesis of 1,25(OH)2D and resulting hypervitaminosis D.

The claims here do not connect the genetic disorders (CYP24A1, SLC34A1) to cancer outcomes, and they do not offer any longitudinal cohort data tracking cancer incidence in people with these mutations over time. The vitamin D–cancer literature and the CYP24A1/SLC34A1 literature come from the same source but describe separate bodies of evidence: one is population-level and observational/RCT, the other is genetic and mechanistic. No claim bridges them.

Where the evidence splits

Observational studies find low serum 25(OH)D associated with increased risk across multiple cancer types, while RCTs of vitamin D supplementation have not shown consistent effects on cancer outcomes. The claims themselves offer a partial explanation: the RCT inconsistency is attributed to short follow-up time, differing supplementation regimens, and different dose levels across trials, while the observational association carries an open question of reverse causality — whether low vitamin D is a cause of cancer risk or a consequence of undiagnosed disease. This is a design-level split (observational association vs. randomized intervention) that the sources acknowledge but do not resolve.

Thresholds and dose-response

No threshold established in the current sources.

Open empirical questions

• What effect size, if any, does vitamin D supplementation produce on cancer incidence once follow-up length, regimen, and dose are standardized across trials?
• Does the low-25(OH)D/cancer-risk association hold after accounting for reverse causality — that is, does undiagnosed disease lower vitamin D status rather than the reverse?
• Does the income-inequality/cancer-mortality association among the 65+ population hold when individual-level income is controlled for, and does it replicate outside a single cross-sectional study?
• Do carriers of CYP24A1 or SLC34A1 mutations show altered long-term cancer risk, and has any longitudinal cohort tracked cancer incidence in this population?