Can Chronic Undereating Cause SHBG of 90 in Men?
Yes, chronic undereating can cause elevated SHBG levels of 90 nmol/L in men, as severe caloric restriction and low body weight are well-established causes of increased SHBG production.
Mechanism of SHBG Elevation with Undereating
Chronic caloric restriction and low body weight increase hepatic SHBG production, creating a metabolic state opposite to obesity where SHBG levels are suppressed 1.
The relationship between nutritional state and SHBG is bidirectional: while obesity and hyperinsulinemia suppress SHBG, undernutrition and low insulin levels have the opposite effect, increasing SHBG production 2, 1.
Insulin and IGF-I normally inhibit SHBG secretion by the liver; in states of chronic undereating, reduced insulin levels remove this inhibitory signal, allowing SHBG to rise 2.
Clinical Context for SHBG of 90 nmol/L
An SHBG level of 90 nmol/L is elevated but within the observed clinical range for men presenting to men's health centers, where values have been documented up to 109 nmol/L 3.
In younger men (≤54 years), only 2.2% have SHBG >60 nmol/L, while in older men (≥55 years), 9% have SHBG >60 nmol/L, making 90 nmol/L uncommon but not rare 3.
The mean SHBG in clinical populations is approximately 32 nmol/L, so a value of 90 represents nearly 3 standard deviations above the mean 3.
Evidence from Weight Loss Studies
Rapid weight loss through very low-calorie diets dramatically increases SHBG: in abdominally obese men, SHBG increased from 27.6 to 48.1 nmol/L during a 9-week very low-calorie diet 4.
This SHBG elevation partially normalizes with weight stabilization but remains elevated above baseline, suggesting that chronic undereating maintains elevated SHBG 4.
Differential Diagnosis to Consider
Before attributing elevated SHBG solely to undereating, evaluate for other common causes:
Hyperthyroidism or subclinical thyroid disease: thyroid hormone directly stimulates SHBG production and is a frequent cause of elevated SHBG 5, 6, 7.
Hepatic disease: early liver disease increases SHBG through impaired metabolism, though SHBG may decline with progression to decompensated cirrhosis 8, 5, 6.
Medications: anticonvulsants, estrogens, and thyroid hormone replacement significantly increase SHBG 5, 6.
Aging: SHBG naturally increases with age, with mean levels of 36.6 nmol/L in men ≥55 years versus 27.7 nmol/L in younger men 3.
HIV/AIDS and smoking: both are associated with elevated SHBG 5, 6.
Clinical Consequences of Elevated SHBG
Elevated SHBG reduces free testosterone availability even when total testosterone appears normal, as more testosterone becomes bound and biologically unavailable 5, 7.
The pituitary senses free testosterone rather than total testosterone, so elevated SHBG can trigger compensatory increases in LH and FSH to maintain adequate free testosterone 5.
Symptoms of hypogonadism may occur despite normal total testosterone when SHBG is markedly elevated 5, 6.
A free testosterone index (total testosterone/SHBG ratio) <0.3 indicates functional hypogonadism and warrants clinical attention 5.
Recommended Evaluation
When encountering SHBG of 90 in a man with suspected chronic undereating:
Measure both total and free testosterone to assess functional androgen status, as total testosterone alone is misleading with elevated SHBG 5, 6.
Check thyroid function tests (TSH, free T4) to exclude hyperthyroidism or subclinical thyroid disease 6.
Evaluate liver function tests even if asymptomatic, as early hepatic disease can elevate SHBG 8, 6.
Review all medications including over-the-counter drugs and supplements 6.
Assess nutritional status comprehensively: body mass index, dietary intake patterns, weight history, and signs of malnutrition 1, 4.
Common Pitfalls to Avoid
Do not rely solely on total testosterone measurements, as this can miss functional hypogonadism caused by elevated SHBG 5, 6.
Do not overlook subclinical thyroid disease, which significantly affects SHBG and is easily treatable 6.
Do not assume undereating is the sole cause without excluding other medical conditions that elevate SHBG 5, 6.