Gynecomastia is the benign enlargement of male breast tissue, resulting from an increase in the glandular component of the chest. This change is primarily driven by an imbalance of sex hormones, specifically an excess of estrogen relative to testosterone. While the condition often affects both sides, presentation on only one side, known as unilateral gynecomastia, is common. This single-sided presentation is a known variation of the condition.
Understanding Gynecomastia and Symptoms
True gynecomastia involves the proliferation of glandular breast tissue. This tissue appears as a firm, rubbery disc located directly beneath the nipple and areola. This growth is a physical manifestation of an altered hormone ratio, where estrogen stimulates the breast tissue. The glandular tissue is distinct from surrounding fat and typically does not decrease with diet and exercise alone.
The presence of this firm mass is often the first symptom and may be accompanied by tenderness or soreness, usually occurring during the early, active phase of tissue growth. It is important to differentiate true gynecomastia from pseudogynecomastia (lipomastia), which is an enlargement caused purely by an accumulation of fat tissue without glandular proliferation. A physical examination can distinguish between the firm glandular tissue of true gynecomastia and the softer, diffuse fatty tissue of pseudogynecomastia.
Is Unilateral Growth Possible
Unilateral growth is a frequent presentation of gynecomastia. Many cases begin on one side before potentially progressing to the other, or they may remain localized to a single breast indefinitely. This asymmetry, where one side is larger than the other, is medically recognized as unilateral or asymmetric gynecomastia.
The reason for this single-sided development is related to localized tissue sensitivity. The hormone receptors within the glandular tissue of one breast may be more sensitive to circulating estrogen levels than the receptors in the other breast. This differential sensitivity causes the tissue on one side to react more strongly to the hormonal stimulus, leading to uneven growth. Thus, even with a systemic hormonal imbalance, the physical manifestation can be asymmetric due to localized tissue biology.
Underlying Drivers of Asymmetric Growth
The underlying drivers of gynecomastia are varied, and their systemic effects can manifest unilaterally due to differences in breast tissue sensitivity. Natural hormonal fluctuations are a common driver, particularly during puberty, where a temporary estrogen increase can cause transient growth that may be more pronounced on one side. Hormonal shifts associated with aging, such as declining testosterone levels, can also trigger unilateral tissue growth in older men.
Medications are a significant category of drivers that disrupt the body’s hormone balance or directly affect breast tissue. Anti-androgens, used for prostate conditions, directly block testosterone’s effects. Certain heart medications, including calcium channel blockers or ACE inhibitors, have also been linked to breast tissue changes. Other classes, such as anti-anxiety drugs, antibiotics, and anabolic steroids, can contribute to the hormonal environment favoring asymmetric growth.
Underlying health conditions that affect hormone metabolism or clearance are another driver. Conditions that impair the liver, such as cirrhosis, or kidney failure can lead to higher circulating estrogen levels because these organs are less efficient at breaking down the hormone. Hyperthyroidism, an overactive thyroid gland, can also alter the balance of sex hormones, sometimes resulting in unilateral enlargement.
In rare instances, unilateral growth may be stimulated by a pathological cause, such as a hormone-secreting tumor. Testicular tumors, for example, can produce hormones that lead to an imbalance, with the effect only noticeable on one side of the chest. Localized trauma or infection is an infrequent, non-hormonal cause of single-sided swelling that should be considered during evaluation.
Medical Evaluation and Next Steps
Any new, unilateral, or progressive breast enlargement warrants a medical consultation to determine the cause and rule out serious conditions. A physician will perform a physical examination to differentiate between true glandular tissue and simple fat, noting the size, texture, and mobility of the mass. The presence of specific “red flag” symptoms makes an urgent evaluation necessary.
Red Flag Symptoms
These concerning signs include:
- Rapid growth.
- Severe or persistent pain.
- Nipple discharge, especially if bloody or occurring only on one side.
- A lump that feels hard, fixed, or is located eccentrically (not directly under the nipple).
The diagnostic process often involves blood tests to check hormone levels (testosterone and estrogen) and to evaluate liver, kidney, and thyroid function. Imaging, such as a breast ultrasound, may also be used to visualize the tissue structure and confirm whether the mass is glandular, fatty, or potentially malignant, as male breast cancer often presents as a unilateral lump.