Thyrotrophin Releasing Hormone (TRH) Test
To assess TSH reserve. Differential diagnosis of pituitary and hypothalamic causes of TSH deficiency.
As patients should be off thyroxine for 3 weeks prior to test so this test, it is rarely used in people on thyroxine.
Overnight fast not necessary.
200 mcg TRH
i.v. cannulae 19 or 21 gauge.
3 x clotted tubes (red top Vacutainers): 6 ml per sample.
Patients should be warned that they may have transient side effects after the injection such as a metallic taste in the mouth, flushing and mild nausea.
- Site indwelling cannula.
- Take baseline bloods for TSH and thyroxine.
- Inject TRH slowly i.v. over 2 minutes.
- Flush butterfly with heparin/saline.
- Take samples for TSH at t = 30 mins and 60 mins.
The normal result is a TSH rise to >5 mU/l with the 30 min value exceeding the 60 min value.
If the 60 min sample exceeds the 30 min value then this usually indicates primary hypothalamic disease.
In hyperthyroidism, the TSH remains suppressed and in hypothyroidism there is an exaggerated response. With the current sensitive TSH assays basal levels are now adequate and dynamic testing is not usually needed to diagnose hyperthyroidism.
SENSITIVITY AND SPECIFICITY
An inadequate rise of TSH is not an indication for thyroxine replacement unless the serum thyroxine, free T4 or free T3 is reduced. The TSH is not only undetectable in pituitary disease and thyrotoxicosis but also in some cases of euthyroid ophthalmic Grave's disease and multinodular goitre.
A late rise in TSH may be seen rarely in thyroid and pituitary disease as well as hypothalamic disease.
Hall et al., Lancet i: 759-63 (1972).