Thyroid-Stimulating Hormone, or TSH, is produced by the pituitary gland. Its primary role is to regulate the production of thyroid hormones, triiodothyronine (T3) and thyroxine (T4), by the thyroid gland. The pituitary gland releases TSH into the bloodstream, which then travels to the thyroid gland and signals it to produce the necessary amount of T3 and T4. A blood test measuring TSH is a common method for assessing thyroid health and function.
TSH Reference Ranges by Age Group
The expected range for TSH levels is not static, changing significantly from birth through old age. These reference ranges are measured in milli-international units per liter (mIU/L). Because the specific values used by laboratories can vary, consulting the reference range provided on a lab report is always recommended.
Immediately after birth, newborns experience a surge in TSH. For a full-term infant up to five days old, a normal range is between 0.7 and 15.2 mIU/L. This level begins to decrease as the infant’s body adjusts to life outside the womb.
For infants and toddlers, the reference range continues to narrow. From six to ninety days old, the range is 0.72 to 11.0 mIU/L. From four to twelve months, the range shifts to 0.73 to 8.35 mIU/L.
Through childhood and adolescence, TSH levels stabilize. For children aged one to six years, the range is 0.7 to 5.97 mIU/L. From ages seven to eleven, the range narrows to 0.6 to 4.84 mIU/L, and for ages twelve to twenty, it is 0.51 to 4.3 mIU/L.
For most adults aged 21 and older, the TSH reference range is between 0.4 and 4.2 mIU/L. As adults enter their later years, TSH levels may increase slightly. For individuals over the age of 80, higher TSH levels may not necessarily indicate a health problem.
Why TSH Levels Fluctuate with Age
TSH levels fluctuate throughout life due to physiological changes and development. The most dramatic shift occurs just after birth. The neonatal TSH surge is a normal response as the infant’s body begins its own thyroid hormone production, a process previously managed through the maternal system.
As a child moves from infancy into adolescence, the endocrine system and the pituitary-thyroid feedback loop continue to mature. This maturation leads to the gradual decrease and stabilization of TSH levels seen throughout childhood as the body establishes a more consistent hormonal balance.
In older adulthood, there is a trend of a slight increase in average TSH levels. It is debated whether this rise is a normal consequence of the aging process, reflecting a slight decrease in thyroid gland efficiency, or if it represents a mild, symptom-free form of underactive thyroid known as subclinical hypothyroidism.
Interpreting Abnormal TSH Results
When TSH levels fall outside the age-appropriate reference range, it can signal an issue with thyroid function. A TSH test is the initial step in diagnosing a thyroid condition, but the results are interpreted with other factors, including levels of thyroid hormones T4 and T3, and the individual’s symptoms.
A TSH level higher than the normal range suggests an underactive thyroid, a condition called primary hypothyroidism. This means the thyroid gland is not producing enough thyroid hormone, so the pituitary gland compensates by releasing more TSH to stimulate it. Common symptoms include:
- Fatigue
- Unexplained weight gain
- Intolerance to cold
- Dry skin
- Constipation
Conversely, a TSH level below the normal range indicates an overactive thyroid, known as hyperthyroidism. This occurs when the thyroid gland produces an excess of thyroid hormone. The pituitary gland senses these high levels and reduces its TSH production accordingly. Symptoms can include:
- Unintentional weight loss
- A rapid or irregular heartbeat
- Anxiety
- Irritability
- An intolerance to heat
Other Factors Influencing TSH Levels
Age is not the only factor that can affect TSH measurements. Several other elements can cause temporary or sustained changes in TSH levels, which is why a single test result may not provide a complete picture.
Pregnancy significantly alters thyroid hormone dynamics. TSH levels decrease during the first trimester, sometimes falling below the standard non-pregnant range, before gradually rising in the second and third trimesters. These shifts are driven by hormones produced during pregnancy, particularly human chorionic gonadotropin (hCG), which can weakly stimulate the thyroid gland.
The time of day a blood sample is taken can also influence the result. TSH secretion follows a diurnal rhythm, with levels at their highest during the night and early morning and lowest in the afternoon. This variation is why consistent timing for follow-up tests can be important.
Certain medications and over-the-counter supplements can interfere with TSH readings. High-dose biotin supplements can lead to falsely low TSH results on some lab assays. Other medications, such as steroids and dopamine agonists, can also impact TSH production. Additionally, an acute or severe illness can temporarily suppress TSH levels, a phenomenon known as non-thyroidal illness syndrome.