Daily Endocrinology Research Analysis
Three impactful endocrinology studies stood out today: a meta-regression of 73,263 participants showed that cardiovascular risk reduction with GLP-1 receptor agonists scales with HbA1c lowering rather than weight loss; a population-based randomized trial found daily high-SPF sunscreen use modestly lowers serum 25(OH)D and increases vitamin D deficiency risk; and the largest to-date LC-MS/MS reference ranges for directly measured free testosterone in men were established, highlighting effects of
Summary
Three impactful endocrinology studies stood out today: a meta-regression of 73,263 participants showed that cardiovascular risk reduction with GLP-1 receptor agonists scales with HbA1c lowering rather than weight loss; a population-based randomized trial found daily high-SPF sunscreen use modestly lowers serum 25(OH)D and increases vitamin D deficiency risk; and the largest to-date LC-MS/MS reference ranges for directly measured free testosterone in men were established, highlighting effects of age and BMI.
Research Themes
- Cardiometabolic therapeutics and outcome surrogates
- Endocrine–dermatology interface (vitamin D and sun protection)
- Androgen assessment and laboratory standardization
Selected Articles
1. Cardiovascular risk reduction with glucagon-like peptide-1 receptor agonists is proportional to HbA1c lowering in type 2 diabetes: An updated meta-regression analysis incorporating FLOW and SOUL trials.
Across 10 placebo-controlled GLP-1RA trials (n=73,263), MACE was reduced by 14%, with additional benefits on heart failure hospitalizations and kidney outcomes. Trial-level meta-regression found cardiovascular benefit proportional to HbA1c reduction, not to weight loss, supporting HbA1c as a potential surrogate for GLP-1RA cardiovascular efficacy.
Impact: Clarifies the mechanistic driver of GLP-1RA cardiovascular benefit and informs endpoint selection and payer policy by prioritizing glycemic improvement over weight change as the key correlate.
Clinical Implications: When prescribing GLP-1RAs for cardiovascular risk reduction, focus on regimens and adherence that maximize HbA1c reduction; weight loss per se may not reflect cardiovascular benefit. HbA1c could be considered a surrogate endpoint for GLP-1RA CV efficacy in decision-making.
Key Findings
- GLP-1RAs reduced MACE by 14% (HR 0.86; 95% CI 0.82–0.91).
- Benefits extended to heart failure hospitalization and composite kidney outcomes (both p<0.001).
- Each additional 1% HbA1c reduction corresponded to a 27% lower HR for MACE; weight change did not predict MACE reduction.
Methodological Strengths
- Inclusion of 10 large placebo-controlled RCTs (n=73,263), including FLOW and SOUL.
- Random-effects meta-analysis with trial-level meta-regression to explore surrogate relationships.
Limitations
- Trial-level (not individual patient-level) meta-regression risks ecological bias.
- Heterogeneity across trials in populations, agents, and follow-up may confound associations.
Future Directions: Conduct individual participant data meta-analyses to validate HbA1c as a surrogate endpoint and quantify thresholds; examine whether early HbA1c response predicts long-term CV benefit across GLP-1RA subclasses.
AIMS: To evaluate relationships of cardiovascular and kidney outcomes with glycemic or bodyweight reductions in randomised placebo-controlled trials of glucagon-like peptide-1 receptor agonists (GLP-1RAs), incorporating data from FLOW and SOUL trials. MATERIALS AND METHODS: PubMed and EMBASE were searched up to 22 August 2025 for placebo-controlled randomized trials of oral or bolus-type, subcutaneous GLP-1RAs reporting major adverse cardiovascular events (MACE; a composite of cardiovascular death, myocardial infarction, and stroke) in adults with type 2 diabetes. The primary outcome was MACE; secondary outcomes included heart failure (HF) and kidney outcomes. Random-effects meta-analyses were followed by meta-regression evaluating associations with HbA1c and bodyweight reduction. RESULTS: A total of 73 263 individuals were included from 10 trials (ELIXA, LEADER, SUSTAIN-6, EXSCEL, Harmony Outcomes, PIONEER 6, REWIND, AMPLITUDE-O, FLOW, and SOUL). GLP-1RAs reduced MACE by 14% (hazard ratio: 0.86; 95% CI: 0.82 to 0.91; p <0.001), as well as hospitalisation for HF and the composite kidney outcome (both p <0.001). Meta-regression showed that every 1% extra reduction in HbA1c corresponded to a 27% lower HR for MACE (p = 0.015; R CONCLUSIONS: HbA1c reduction, not bodyweight change, was significantly and proportionally associated with MACE risk reduction. HbA1c lowering may serve as a useful surrogate for the cardiovascular improvements associated with GLP-1RAs in type 2 diabetes.
2. Effect of daily sunscreen application on vitamin D: findings from the open-label randomized controlled Sun-D Trial.
In a population-based open-label RCT (n=639), routine SPF50+ use on UV index ≥3 days for ~1 year led to lower 25(OH)D compared with discretionary use (between-group effect −5.2 nmol/L; 95% CI −7.2 to −3.2) and a higher prevalence of vitamin D deficiency (PR 1.33). Effects were consistent across UV zones and exposure strata.
Impact: Provides high-quality randomized evidence that routine high-SPF sunscreen use modestly lowers vitamin D status in real-world settings, informing public health guidance on supplementation while maintaining skin cancer prevention.
Clinical Implications: Counsel regular sunscreen users—especially at higher latitudes or with baseline insufficiency—regarding vitamin D testing and supplementation while continuing sun protection. Integrate vitamin D management into dermatology and primary care prevention plans.
Key Findings
- Routine SPF50+ use produced a smaller change in 25(OH)D vs. discretionary use (treatment effect −5.2 nmol/L, 95% CI −7.2 to −3.2).
- Vitamin D deficiency at final sampling was more frequent with routine sunscreen (45.7% vs. 36.9%; PR 1.33, 95% CI 1.14–1.55).
- Effects were consistent across baseline vitamin D, UV radiation zone, skin exposure, and personal UVR exposure strata.
Methodological Strengths
- Population-based randomized allocation with stratified permuted blocks; pre-registered protocol.
- Repeated 25(OH)D measurements across seasons with subgroup stratification by UV exposure.
Limitations
- Open-label design may allow behavioral differences beyond sunscreen use.
- Findings may be context-specific to Australian UV patterns and may not generalize to all latitudes.
Future Directions: Evaluate optimal vitamin D supplementation strategies for regular sunscreen users in diverse climates; test targeted screening thresholds based on baseline 25(OH)D and UV exposure patterns.
BACKGROUND: Sunscreen reduces vitamin D production in experimental studies. It is uncertain whether this translates to real-world settings. OBJECTIVES: To determine if routinely applying high sun protection factor (SPF) sunscreen for a year reduces serum 25-hydroxyvitamin D [25(OH)D] concentration. METHODS: We conducted a population-based open-label randomized controlled trial in Australian participants aged 18-70 years who were not routinely using sunscreen or taking vitamin D supplements. Participants were randomized (1 : 1) using stratified, computer-generated permuted block randomization to routine application of SPF 50+ sunscreen on days the ultraviolet (UV) index was forecast to reach ≥ 3 (intervention) or discretionary sunscreen use (control), for approximately 1 year. We measured 25(OH)D concentration at baseline (winter/spring 2022) and at the end of summer and winter 2023; participants with no postbaseline concentrations (n = 11) were excluded from analyses. The primary outcome was change in 25(OH)D concentration from baseline. Vitamin D deficiency [25(OH)D concentration < 50 nmol L-1] in the final sample was an exploratory outcome. We analysed data using mixed-effects models and Poisson regression. We stratified our analysis by baseline 25(OH)D concentration, residential UV radiation (UVR) zone, skin exposure (derived from time outdoors and clothing use) and personal UVR exposure (derived by combining UVR zone and skin exposure). Sample and data analysis were performed blind to randomization group. The trial was registered with the Australian New Zealand Clinical Trials Registry (ACTRN12621001752853). RESULTS: Between 30 June and 29 November 2022, we randomized 639 participants (intervention, n = 319; control, n = 320). In total, 628 participants were analysed [intervention, n = 312; control, n = 316; median age 52 years (interquartile range 40-64)]. Altogether, 415 (66.1%) identified as female, 210 (33.4%) as male and 3 (0.5%) used another term. Mean (SD) baseline 25(OH)D concentration was balanced [intervention 63.5 (21.9) nmol L-1; control 62.1 (22
3. Age-Stratified Reference Ranges for Directly Measured Serum Free Testosterone in Community-Dwelling and Healthy Men.
Using equilibrium dialysis with LC-MS/MS in 1,202 men from population cohorts, the study established robust age-stratified reference ranges for directly measured free testosterone. mFT declines by ~4.5 pmol/L per year and is 14–22% lower at BMI 30–35 vs. BMI 22, informing interpretation of hypogonadism workups.
Impact: Provides the largest, methodologically rigorous reference ranges for directly measured FT using the gold-standard technique, addressing a major gap that affects diagnosis of male hypogonadism.
Clinical Implications: Adopt age- and BMI-aware interpretation of directly measured FT; consider using these ranges to calibrate clinical thresholds and to refine when calculated FT is sufficient versus requiring direct measurement.
Key Findings
- Established mFT reference range for healthy, non-obese men aged 18–39 years: 184–749 pmol/L.
- mFT decreases with age (~−4.5 pmol/L per year).
- Higher BMI (30 and 35 kg/m²) associates with 14.4% and 22.2% lower mFT vs. BMI 22 kg/m².
Methodological Strengths
- Gold-standard measurement via equilibrium dialysis followed by LC-MS/MS.
- CLSI-guideline–based approach to derive age-stratified reference intervals in the largest cohort to date.
Limitations
- Predominantly Caucasian men limit generalizability across ethnicities.
- Cross-sectional design does not link reference ranges to clinical outcomes.
Future Directions: Validate ranges across diverse ethnicities and clinical states; evaluate diagnostic cut-points against clinical outcomes and explore integration with calculated FT algorithms.
CONTEXT: Direct measurement of serum free testosterone (FT) may help evaluate hypogonadism in men. However, up to present, availability of reference ranges for measured FT (mFT) is limited. OBJECTIVE: To establish age-stratified reference ranges for mFT in healthy and community-dwelling adult men. METHODS: Serum samples of 1202 Caucasian men were collected from large population cohort studies. Free testosterone was measured using equilibrium dialysis followed by liquid chromatography tandem-mass spectrometry (LC-MS/MS). Sex-hormone binding globulin was measured using immunoassay and total testosterone was measured using LC-MS/MS. Reference ranges per age decade and for men aged 18 to 39 years with BMI < 30 kg/m² were established following Clinical & Laboratory Standards Institute guidelines. Reference ranges were established as the 2.5th and 97.5th percentiles. RESULTS: The reference range for mFT in healthy, non-obese men aged 18 to 39 years was 184-749 pmol/L. mFT levels showed a decrease with aging of -4.5 pmol/L per year. Having a BMI of 30 kg/m² and 35 kg/m² resulted in a decrease in mFT of 14.4% and 22.2%, respectively, compared to a reference BMI of 22 kg/m². CONCLUSION: This study provides age-stratified reference ranges for mFT in adult men, established in the largest cohort to date, using published methodology. Our results also show that mFT levels are influenced by age and BMI. Future work should focus on whether age-stratified or reference ranges for young men should be employed, and how mFT can best be implemented in clinical practice including the improved application of calculated FT.