Harmonized reference normal ranges for testosterone levels in healthy, nonobese men have been generated in a landmark study that should aid in accurately diagnosing hypogonadism, say US investigators.

They say their findings underline the variable performance of hormone assays.

The diagnosis of hypogonadism, which is characterized by low testosterone levels and can lead to sexual dysfunction, decreased bone and muscle strength, and lower fertility, relies on accurate hormonal measurements. However, the task is made more difficult by a lack of both defined references ranges for testosterone and standardized assays.

Harmonizing testosterone levels from four major cohort studies, Shalender Bhasin, MD, from Brigham and Women's Hospital, Harvard Medical School in Boston, Massachusetts, and colleagues were able to identify a reference range for testosterone that could help clinicians in decision making and improve patient care — this was in the region of 250 to 900 ng/dL for 19- to 39-year-old men.

The new findings, which are based on measurements taken from over 9000 men and calibrated against a US Centers for Disease Control and Prevention (CDC) standardized assay, offer, for the first time, a reliable baseline for total testosterone levels.

"Well-defined reference ranges are at the heart of clinical practice and, without them, clinicians can make erroneous diagnoses that could lead to patients receiving costly, lifelong treatments that they don't need or deny treatments to those who need them," said Dr Bhasin said in an Endocrine Society press release.

While highlighting that the findings establish a reference range for testosterone, he added: "These data also show that variations in assays are an important contributor to variation in testosterone levels in cohorts from different geographic regions. Clearly, we need standardization in all hormone assays."

The work, which was published online in the Journal of Clinical Endocrinology & Metabolism on January 10, was cofunded by the Endocrine Society.

Coauthor Hubert Vesper, PhD, who is cochair of the Partnership for the Accurate Testing of Hormones (PATH), an Endocrine Society initiative, commented, "Without harmonized reference ranges and standardized assays, tests can lead to misdiagnoses, and unfortunately this happens every day around the world."

He emphasized that it is important that the new reference range for testosterone be taken "into consideration in the tests that clinicians and patients depend on for accurate diagnoses."

Another recent study has suggested that measuring free testosterone alongside total testosterone may offer a more accurate picture when diagnosing hypogonadism.

As reported by Medscape Medical News, analysis of 3000 men in eight European countries indicated that men with normal total testosterone but low free testosterone levels had more signs and symptoms of hypogonadism than those with low total-testosterone levels only.

In the current research, to identify harmonized reference ranges for testosterone in men, Dr Bhasin and colleagues compared the distribution of serum total testosterone concentrations from four epidemiological studies in Europe and the United States: the Framingham Heart Study (FHS); European Male Aging Study; Osteoporotic Fractures in Men Study (MrOS); and Male Sibling Study of Osteoporosis. Most of the participants were of European descent.

This yielded a cohort of 9054 community-dwelling adult men with serum testosterone levels. As each study had used different assays for measuring testosterone, with each assay using different calibrators, the team obtained fasting morning serum samples from 100 men in each cohort, which were then analyzed in the CDC Clinical Reference Laboratory using a single, highly calibrated assay.

Comparing the CDC-derived values with those from the original studies, the team developed normalizing equations to generate harmonized values.

As testosterone levels decline with age, the researchers initially generated reference ranges in healthy, nonobese men aged 19 to 39 years, followed by ranges stratified by decade of age.

The harmonization process was able to substantially reduce the intercohort variation in testosterone levels in men of similar ages, with the largest decreases seen in the FHS cohort and the largest increases in the MrOS cohort.

The researchers calculated that, in healthy, nonobese men aged 19 to 39 years, CDC-standardized testosterone levels were 264 ng/dL for the 2.5th percentile, 303 ng/dL for the 5th percentile, 531 ng/dL for the 50th percentile, 852 ng/dL for the 95th percentile and 916 ng/dL for the 97.5th percentile.

Thus, the harmonized normal range for testosterone levels in this age group [19–39 years] using the CDC standardized test was 264 to 916 ng/dL, while that for all men aged 19 to 39 years, including those who were obese, was 228 to 895 ng/dL.

For older age groups, among all men (including the obese) the age-specific estimates for normal reference ranges of testosterone were: 208 to 902 ng/dL for age 40 to 49 years, 192 to 902 ng/dL for 50 to 59 years, 190 to 902 ng/dL for both 60 to 69 years and 70 to 79 years, and 119 to 902 ng/dL for 80 to 99 years.

The team says: "We conclude that standardized hormone measurements calibrated to a higher-order benchmark, such as that offered by the CDC Clinical Reference Laboratory, provide a rational and feasible approach to generating harmonized reference ranges for testosterone and possibly other analytes."

They note that previous experience with assays for cholesterol, HbA 1c , and vitamin D suggests that "the application of reference ranges across laboratories and across geographic regions is a challenging process that requires mechanisms for standardizing assays and an understanding of biological as well as social differences in the distribution of the analyte.

"The CDC Hormone Standardization Program for testosterone is an important step to address this challenge, which will facilitate the application of these reference ranges across laboratories."

They conclude: "Further studies of the distribution of testosterone concentrations in other racial and ethnic groups and in populations in other regions of the world are needed to demonstrate the applicability of these ranges to broader populations of men in different regions of the United States and the world."

This study was supported primarily by an National Institute Grant to Dr Bhasin. Additional support was provided by the Endocrine Society and the Boston Claude D Pepper Older Americans Independence Center grant from the National Institute on Aging. Dr Bhasin has received research grant support from AbbVie Pharmaceuticals, Transition Therapeutics, Takeda Pharmaceuticals, and Eli Lilly for investigator-initiated research unrelated to this study. He has served as a consultant to AbbVie, Regeneron, Novartis, and Eli Lilly and has a financial interest in Function Promoting Therapies, a company aiming to develop innovative solutions that enhance precision and accuracy in clinical decision making and facilitate personalized therapeutic choices in reproductive health. His interests were reviewed and are managed by Brigham and Women's Hospital and Partners HealthCare in accordance with their conflict-of-interest policies. Dr Vesper has no relevant financial relationships. Disclosures for the coauthors are listed in the paper.

For more diabetes and endocrinology news, follow us on Twitter and on Facebook.

J Clin Endocrinol. Published online January 10, 2017. Abstract