Thyrotropin releasing hormone (TRH) test
The development of TSH assays capable of accurate measurements below 0.1 mU/L has obviated the need for the TRH test in most cases of thyroid practice except perhaps in the differential diagnosis of TSHoma and thyroid hormone resistance (high TSH and high thyroxine).
TRH can cause smooth muscle spasm and should be used with caution in patients with asthma or ischaemic heart disease. The TRH test should not be used in pregnant women. Patients should not have taken thyroxine for 3 weeks prior to this test.
TRH (thyrotropin releasing hormone) is a tripeptide secreted by the hypothalamus which stimulates the production and secretion of TSH by the anterior pituitary. TRH stimulates prolactin release.
Most adult patients express an urgent need but inability to pass urine. Other side effects include flushing, dizziness and a metallic taste in the mouth.
No specific patient preparation is required
- 3 serum tubes
- TRH 200 microgm (adults). The dose for children is 7 microgm/kg to a max 200 microgm.
time 0 min
take 3 mL blood
immediately give TRH iv as a bolus (dose as above)
time 20 min
take 3 mL blood
time 60 min
take 3 mL blood
- Normal basal values of TSH should be 0.2-6 mU/L. The normal increment in TSH at 20 min should be 5-30 (mean 15) mU/L with a slight diminution at 60 min.
- Exaggerated TSH response is seen in primary hypothyroidism.
- A flat response is seen in primary hyperthyroidism; but also in some apparently euthyroid patients with ophthalmic Graves disease or multinodular goitre.
- A delayed response with the TSH concentration lower at 20 than 60 min may be seen in hypothalamic dysfunction.
- Various drugs can modify the TSH response: it is reduced by glucocorticoids, DOPA agonists eg L-DOPA, bromocryptine and fluoxetine; and enhanced by DOPA antagonists eg metoclopramide, oestrogens and theophylline and sertraline (but ? only in thyroxine treated subjects).
- In neonates, peak TSH responses < 35mU/L are not associated with subsequent hypothyroidism, whereas responses > 35 mU/L are associated with a rate of subsequent hypothyroidism of 35%.
- The TSH response is flat in most cases of TSHoma whereas in thyroid hormone resistance the TSH response is brisk.
- Baylis PH. Drug induced endocrine disorders. Adverse Drug Reaction Bull 1986;116:432-435.
- Brucker-Davis F, Ildfield EH, Skarulis MC, Doppman JL, Weintraub BD. Thyrotropin-secreting pituitary tumours: diagnostic criteria, thyroid hormone sensitivity, and treatment outcome in 25 patients followed at the National Institutes of Health. J Clin Endocrinol Metab 1999;84:476-486.
- Casanueva FF, Webb SM, Dieguez C. Thyrotrophin-secreting pituitary tumours. In: Clinical Endocrinology ed Grossman A. Blackwell Science, 2nd edit 1997:204-215.
- Pijl H, Koppeschaar HP, Willekens FL, Frolich M, Meinders AE. The influence of serotonergic neurotransmission on pituitary hormone release in obese and non-obese females. Acta Endocrinologica 1993;128:319-324.
- Rapaport R, Sills I, Patel U, Oppenheimer E, Skuza K, Horlick M, Goldstein S, Dimartino J, Saenger P. Thyrotropin-releasing hormone stimulation in infants. J Clin Endocrinol Metab 1993;77:889-894.