From the Guidelines
Thyroid function tests should be interpreted with caution, considering the variability of TSH secretion and the lack of consensus on normal reference intervals.
Key Considerations
- TSH levels can be influenced by various factors, such as acute illness, certain medications, and physiological conditions like pregnancy or adrenal insufficiency 1.
- There is no consensus on the appropriate TSH cutoff for a diagnosis of subclinical hypothyroidism or hyperthyroidism, with most laboratories using the upper and lower limits of the 95% reference interval for a particular assay 1.
- Serial TSH measurements are essential to establish a diagnosis of thyroid dysfunction, as a single abnormal value may not be reliable due to day-to-day variability of up to 50% of mean values 1.
Interpretation Guidelines
- A TSH level greater than 10.0 mIU/L is generally considered the threshold for initiation of treatment, as it is associated with a higher likelihood of progression to overt thyroid dysfunction 1.
- TSH levels between 4.5 and 10.0 mIU/L are more controversial, and the decision to begin therapy should be made on a case-by-case basis, considering the individual's clinical symptoms and risk factors 1.
- Repeating thyroid function tests over a period of 3-6 months is recommended to confirm persistent dysfunction before making a diagnosis or considering treatment strategies, unless the serum TSH level is greater than 10.0 or less than 0.1 mIU/L 1.
From the Research
Interpretation of Thyroid Function Tests
The interpretation of thyroid function tests (TFTs) is generally straightforward, but can be confusing in some cases due to discordant results or unusual patterns 2, 3. To accurately interpret TFTs, it is essential to consider the clinical context and potential confounding factors, such as:
- Alterations in normal physiology (e.g., pregnancy)
- Intercurrent (non-thyroidal) illness
- Medication usage (e.g., thyroxine, amiodarone, heparin)
Common Pitfalls in TFT Interpretation
Some common pitfalls in TFT interpretation include:
- Laboratory artefacts in TSH or TH immunoassays
- Interference from anti-thyroid hormone antibodies
- Familial dysalbuminemic hyperthyroxinemia
- Rare genetic and acquired disorders of the hypothalamic-pituitary-thyroid (HPT) axis (e.g., resistance to thyroid hormone, thyrotropinoma) 2, 4
Role of TSH and Free Thyroid Hormones in TFT Interpretation
TSH and free thyroid hormones (FT4 and FT3) are essential for confirming and assessing the level of hypo- and hyperthyroidism 4. In cases where the thyroidal status is unstable or the hypothalamic-pituitary function is disturbed, TSH determination may be diagnostically misleading, and only free hormone measurements are reliable for thyroid function assessment 4. A study comparing the variation in TSH and T4 in two different cohorts found that TSH outperforms T4 in detecting delicate differences in thyroid function in an individual 5.
Importance of Considering Interference and Carrier Proteins
When interpreting thyroid hormone test results, it is crucial to consider interference and changes in concentration of their carrier proteins 6. This can help avoid erroneous diagnosis of thyroid disorders and potentially harmful treatment. For example, heterophile antibodies can lead to false lower TSH levels, and low thyroxine-binding globulin (TBG) can result in low total thyroid hormones with normal TSH 6.