From the FDA Drug Label
Monitor serum free-T4 levels and maintain in the upper half of the normal range in these patients. The labs used to diagnose secondary hypothyroidism are:
- Serum free-T4 levels To diagnose and monitor secondary hypothyroidism, serum free-T4 levels should be maintained in the upper half of the normal range 1.
From the Research
To diagnose secondary hypothyroidism, laboratory tests should include both thyroid stimulating hormone (TSH) and free thyroxine (free T4) levels, as the hallmark finding is a low or inappropriately normal TSH level combined with a low free T4 level, as noted in studies such as 2. When evaluating thyroid function, it's essential to consider the most recent and highest quality studies. According to 2, measurement of serum free thyroxine index may provide additional case detection compared to free thyroxine in the diagnosis of central hypothyroidism.
Key Laboratory Tests
- Thyroid stimulating hormone (TSH)
- Free thyroxine (free T4)
- Free triiodothyronine (free T3) may also be useful, especially if TSH is undetectable and free T4 is normal, as suggested by 3
- Assessment of other pituitary hormones such as cortisol, luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin to evaluate for broader pituitary dysfunction
Additional Diagnostic Tools
- Morning cortisol to assess for adrenal insufficiency, which may coexist and should be treated before thyroid replacement
- Pituitary MRI to identify structural abnormalities like tumors or infarction that might be causing the secondary hypothyroidism
- Thyrotropin-releasing hormone (TRH) stimulation testing, though less commonly used, can help distinguish between pituitary (secondary) and hypothalamic (tertiary) causes by assessing the TSH response to TRH administration, as mentioned in 3. It's crucial to prioritize the most recent and highest quality studies when making diagnostic decisions, and in this case, 2 provides valuable insights into the diagnosis of central hypothyroidism.