From the Guidelines
A TSH level greater than 2.5 mU/L is not necessarily indicative of uncontrolled hypothyroidism, as the normal range for TSH typically extends from about 0.4 to 4.0-4.5 mU/L in most laboratories. The reference range for normal TSH levels can vary between laboratories, but a level between 2.5 and 4.5 mU/L is generally considered within the normal range for most adults 1. True hypothyroidism is typically diagnosed when TSH exceeds the upper limit of normal (>4.0-4.5 mU/L) along with low or low-normal free T4 levels.
Key Points to Consider
- For patients already being treated for hypothyroidism, the target TSH range is usually 0.5-2.5 mU/L, though this can vary based on individual factors such as age and comorbidities 1.
- During pregnancy, stricter TSH targets are recommended (below 2.5 mU/L in the first trimester and below 3.0 mU/L in later trimesters).
- TSH levels should always be interpreted in the context of clinical symptoms, other thyroid function tests, and the patient's overall health status rather than relying on a single value.
Clinical Implications
The decision to initiate or adjust treatment for hypothyroidism should be based on a comprehensive evaluation of the patient's clinical status, rather than relying solely on TSH levels. A TSH level greater than 10.0 mIU/L is generally considered the threshold for initiation of treatment, as it indicates a higher likelihood of progression to overt thyroid dysfunction 1. However, the decision to begin therapy in patients with TSH levels between 4.5 and 10.0 mIU/L is more controversial and should be individualized based on the patient's specific circumstances.
From the FDA Drug Label
The general aim of therapy is to normalize the serum TSH level In pregnant patients with primary hypothyroidism, maintain serum TSH in the trimester-specific reference range
The normal TSH level range is not explicitly stated in the provided drug labels as 0.5 to 1. A TSH level greater than 2.5 may indicate uncontrolled hypothyroidism or inadequate replacement therapy, but the exact threshold for this indication is not specified in the labels. Key points to consider:
- The general aim of therapy is to normalize the serum TSH level.
- In patients with hypothyroidism, monitor TSH levels after any change in dosage.
- Trimester-specific reference ranges should be used for pregnant patients. 2
From the Research
Normal TSH Levels
- The normal TSH level is not strictly 0.5 to 1, as previously stated. According to a study published in 2025 3, the median TSH level in women is 1.70 mIU/l, with a range of 0.37-6.95 mIU/l.
- Another study from 2021 4 suggests that TSH levels within the normal laboratory range can still have an impact on health-related quality of life (HRQoL) in patients with primary hypothyroidism.
TSH Levels and Hypothyroidism
- A TSH level greater than 2.5 mU/L may indicate uncontrolled hypothyroidism, as suggested by a study published in 2025 5, which found that Japanese thyroid specialists would prescribe thyroid hormones to euthyroid subjects with TSH levels between 2.5-4 mU/L.
- However, it's essential to consider that TSH levels can vary depending on several factors, including age and individual characteristics, as noted in the study published in 2025 3.
- The study from 2021 4 also found that higher TSH values, even within the normal reference range, are associated with greater deterioration of HRQoL in patients with primary hypothyroidism.
Treatment of Hypothyroidism
- Levothyroxine (LT4) is the established treatment for hypothyroidism, as stated in the study published in 2020 6.
- Some studies suggest that combining LT4 with liothyronine (LT3) may be beneficial for patients with residual symptoms or low serum T3 levels, as noted in the studies published in 2020 6 and 2025 5.
- However, the use of LT3 + LT4 combination is less frequent in Japan than in other countries, as reported in the study published in 2025 5.