The result of 1.39 milli-international units per liter (mIU/L) for your Thyroid-Stimulating Hormone (TSH) test is a numerical indicator of your thyroid gland’s current function. The TSH test is the most sensitive single measure of the thyroid system and reflects the complex hormonal balance within the body. For most healthy adults, a result of 1.39 mIU/L is considered a positive finding that suggests a well-regulated thyroid.
What TSH Measures
Thyroid-Stimulating Hormone is a glycoprotein hormone produced by the pituitary gland, located at the base of the brain. Its primary function is to signal the thyroid gland to produce and release its hormones, primarily thyroxine (T4) and triiodothyronine (T3). These hormones regulate the body’s metabolism, energy levels, and heart function.
The system operates via a negative feedback loop, similar to a thermostat. When T4 and T3 levels drop too low, the pituitary increases TSH production to stimulate the thyroid. Conversely, if the thyroid releases too much T4 and T3, the pituitary reduces TSH output to slow its activity. Therefore, a TSH measurement is an inverse reflection of the amount of functional thyroid hormone circulating.
Interpreting the 1.39 Result
For healthy, non-pregnant adults, the standard reference range for TSH is approximately 0.4 to 4.5 mIU/L, though this can vary slightly between laboratories. Your result of 1.39 mIU/L falls comfortably within this wide normal range, indicating that your thyroid system is functioning appropriately without over- or under-producing hormones.
Many clinicians consider a narrower window of 1.0 to 2.5 mIU/L to be the optimal range for individuals not on thyroid medication. The 1.39 result places you within this target zone, which is often associated with efficient metabolic function. Results near this central point are viewed as ideal, especially when accompanied by an absence of common thyroid-related symptoms like unexplained weight change, persistent fatigue, or issues with body temperature regulation.
The 1.39 value suggests a state of euthyroidism, meaning a healthy balance of thyroid hormone. A TSH test is typically interpreted alongside measurements of free T4 and free T3 to fully assess thyroid status. If you are taking thyroid hormone replacement medication, a TSH level of 1.39 mIU/L indicates that your current dosage is likely effective and maintaining your thyroid function in the desired therapeutic range. Interpretation of any TSH result must always be contextualized by a person’s medical history and current symptoms.
Why TSH Results Change
Even when a TSH result is optimal, TSH levels are not static and are subject to natural physiological fluctuations. The pituitary hormone follows a distinct circadian rhythm, meaning its concentration changes throughout the day, tending to be lowest in the afternoon and highest during the night or early morning.
Various non-pathological factors can also influence a TSH reading from one test to the next. TSH levels tend to increase slightly as a person ages. Certain common medications and supplements are known to interfere with the TSH assay or the hormone’s metabolism in the body.
Acute illness or emotional stress can temporarily alter the complex feedback loop, causing a transient shift in the TSH value. Pregnancy is another major physiological factor, as the TSH target range changes significantly, often dropping lower during the first trimester. For instance, high doses of the supplement Biotin can artificially lower the TSH reading in a lab test if taken too close to the blood draw.