Endocrine Dynamic Function Tests



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Endocrine Dynamic Function Tests


THYROTROPHIN-RELEASING HORMONE (TRH) TEST

Indications:

  1. Historically, the TRH test was important in defining subtle degrees of Hypo- and Hyperthyroidism. With the advent of high sensitivity TSH measurements and redefined robust assays for FT3 and FT4 the role of the TRH test is limited.

  2. As part of Combined Anterior Pituitary Function Testing in the work-up of hypothalamic pituitary lesions.

  3. Rarely, to aid in diagnosis of a TSH secreting tumour (TSH-oma)

Contra-indications:

Avoid in early pregnancy



Precautions:

Transient pressor effects, variations in blood pressure and tachycardia can occur. Best avoided in those with coronary artery disease, hypertension, obstructive airways disease or asthma. TRH has been implicated as a cause of pituitary apoplexy in patients with pituitary adenomas.



Side Effects:

Transient nausea, metallic taste, desire to micturate, flushing and elevation of systolic and diastolic blood pressure.



Patient Preparation:

Withdraw Thyroid hormone replacement treatment one month prior to testing.

Otherwise, no special patient preparation required. No requirement for fasting.

Procedure:

1) Insert indwelling cannula, 30 mins prior to the first blood sample.

2) Take blood for basal FT4 and TSH measurement (single SST, Time 0).

3) Inject 200 µg TRH as a rapid iv bolus.

4) Take further blood samples for TSH at 30 and 60 mins (single SST each time).

5) Send all specimens to the laboratory at the end of the test together with the completed proforma.



Guidelines to Interpretation:

Normal response: TSH rises to between 5 and 15mU/l, with 30 min value > 60 min value.

Patients with Hypothyroidism have an elevated TSH by definition and the test is therefore superfluous in this situation. If carried out, a marked rise in TSH is seen.

Patients with destructive Pituitary lesions (hypopituitarism) show an absent or diminished response to TRH.

In patients with Thyrotoxicosis, including Euthyroid Opthalmic Graves disease, the basal TSH is suppressed and fails to respond to TRH.

Patients with TSH-oma have an elevated basal TSH but show no response to TRH.

Patients with Hypothalamic disease show a delayed peak response with the TSH value at 60 mins exceeding that at 30 mins.

Please note that these guidelines apply to adults only; for children please contact Dr Amanda Billson, Consultant Paediatrician




Protocol enquiries: 01225 824711

Department of Clinical Biochemistry, RUH






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