Low or normal TSH, low free thyroid hormones
This combination is most frequently seen in patients with other non-thyroid illness.
In the patient who is not systemically unwell then the low TSH may reflect pituitary pathology with secondary hypothyroidism. It is important to make this diagnosis because:
- the associated hypoadrenalism may be fatal
- pituitary tumours may cause severe local damage
The investigation should include:
- short Synacthen test
- FSH, LH and sex steroid determinations
- MRI of the brain
Patients who have recently been treated for hyperthyroidism may have residual suppression of TSH levels for up to a year. Such patients who have low TSH and low free thyroid hormones are hypothyroid and should be treated as such.
Reference
- Thyroid Function Tests. British Thyroid Foundation, revised 2021
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