Comparing temperature readings from different body parts often reveals variations that lead to confusion. An axillary (armpit) temperature reading being higher than an oral reading is particularly unusual, as the opposite is generally expected. Understanding how different sites estimate the body’s internal warmth reveals that such a discrepancy is nearly always the result of temporary, outside influences. This explanation details the standard relationship between these two measurements and the specific factors that can cause this reversal.
Understanding Core Body Temperature and Measurement Sites
The goal of any temperature measurement is to estimate the body’s core temperature, which is the regulated warmth of the vital organs like the brain and heart. This core temperature is highly stable and represents the body’s true internal state. Since directly measuring the core is invasive, all common methods rely on peripheral sites that reflect this internal warmth.
The oral site is considered a semi-internal measurement because the thermometer is placed in the sublingual pocket, a highly vascular area under the tongue. This location is a relatively protected cavity that receives blood directly from the internal carotid artery, allowing it to closely mirror the temperature of the blood circulating toward the brain. For this reason, the oral reading is typically a good, non-invasive proxy for core body temperature.
The axillary site, however, is a measurement of shell or surface temperature, which is the warmth of the skin and subcutaneous tissues. The armpit area is an open skin surface, meaning its temperature is easily influenced by the surrounding environment and peripheral blood flow. Since the armpit is less protected than the mouth and is prone to heat loss through evaporation, its reading is inherently less precise as an estimate of core temperature.
Standard Differences Between Oral and Axillary Readings
Under normal, resting conditions and with proper technique, an axillary reading is expected to be lower than an oral reading. This consistent difference exists because the oral cavity is insulated and reflects the warmth of major blood vessels. The axilla, conversely, is an outer body surface that constantly loses heat to the environment.
The standard difference observed in clinical practice is typically between 0.5°F and 1.0°F (0.3°C to 0.6°C), with the armpit measurement being the lower value. If an oral thermometer reads 98.6°F (37.0°C), a simultaneous axillary reading would likely be closer to 97.6°F (36.4°C). When a temperature reading deviates from this expected relationship, it suggests that external factors have artificially altered one or both measurements.
Factors That Skew Axillary Readings Upward
The most common reason for an axillary reading to appear unusually high is poor measurement technique that traps heat. The thermometer probe must be placed directly in the center of the armpit, touching skin only, with the arm held snugly against the body for the full measurement duration. If the probe is not adequately secured in this central pocket, or if the contact time is insufficient, the reading can be inaccurate.
Environmental and behavioral factors also contribute to an artificially elevated axillary temperature. Recent vigorous physical activity, such as intense exercise or friction from quickly rubbing the arm, can increase peripheral circulation and local skin temperature. Furthermore, being bundled in heavy blankets, wearing multiple layers of clothing, or sitting in a warm room before the measurement can prevent normal heat dissipation from the armpit. To avoid this artificial elevation, one should wait approximately 20–30 minutes after these warming activities before taking a reading.
Factors That Skew Oral Readings Downward
In many cases where the axillary temperature seems high, the oral temperature may actually be artificially low, narrowing or reversing the expected temperature gap. The oral site is extremely sensitive to recent ingestion and respiration patterns. Consuming cold liquids or foods immediately before a measurement can rapidly cool the highly vascular tissues under the tongue.
To ensure an accurate oral reading, it is recommended to wait a minimum of 15 to 30 minutes after drinking or eating anything. Mouth breathing, especially if it is deep or rapid, allows cool air to flow over the mucous membranes, suppressing the temperature of the oral cavity.
Smoking immediately before using the thermometer can lower the surface temperature inside the mouth, resulting in a deceptively low reading. Finally, improper placement of the thermometer, such as letting the probe rest on the side of the mouth instead of deep in the heat pocket beneath the tongue, can also cause the oral reading to be falsely decreased.