TRH test

Indications: 

1.To diagnose preclinical primary hypothyroidsm who show an exaggerated TSH
response.

2.In those with central hypothyroidsm allowing the differentiation between pituitary 
and hypothalamic hypothyroidsm.

3.May be used in the diagnosis of subclinical primary hyperthyroidsm.(generally
unnecessary since the introduction of ultra sensitive TSH assays) but may be
necessary where baseline TSH is variable.

4.Added test in hyperprolactinemia to discriminate functional hyperprolactinemia
from microprolactinoma.(not done routinely especially with the availability of MRI).

5.To distinguish between TSHomas (usually unresponsive) and Thyroid resistance 
states.(always responsive)

6. Helpful alternative test in acromegaly with Growth Hormone measurements when 
other results are equivocal.


Patient preparation:

1. No fasting necessary. Light breakfast (tea / toast)
2. Weigh patient
3. Baseline BP and pulse.
4. Patients should be off thyroxine for 3 weeks prior to test, though this test is very
rarely used on patients taking thyroxine


Dose:

0.2mg TRH iv
Measure baseline T4 and TSH at time 0 and TSH at 20, 60 min +/- prolactin/GH

Precautions: (better not to do)

 Coronary artery disease.

Side effects: 

Mild and transient. nausea, desire to micturate, flushing, dizziness and peculiar
taste.Lasts a few minutes, best done when lying supine.


Interpretation:

Normal response is rise in TSH by more than 2 mu/l to > 3.4 mu/l but < 22 mu/l with a maximum at 20 min and lower values at 60 min.

Typical responses in thyroid disease:

Time	Normal	甲狀腺低下	甲狀腺亢進	hypopituitarism	Hypothalamic defect
0 min	0.4-4	5-50	0.1	1	1
20 min	5-22	22-90	0.1	3	4
60 min	3-15	15-80	0.1	2	7

These are rough guides only and gives a pattern of defects than absolute values.
In Ninewells the cutoff is taken at 22 mu/l of TSH for primary preclinical hypothyroidsm.

Oestrogens, theophylline and levodopa enhance the response
Steroids depress response.

2.If the rise in prolactin is <100 % , then this is thought to indicate a tumor. (prolactinoma不太受TRH刺激)
In normal and hyperfunctional states prolactin rise is more than 100% (usually 3 –5 fold)

3.The normal GH response at 20 and 60 min following TRH is GH suppression,
however 80% of patients with acromegaly show an increase (by 50% of basal).


Generally:

1. A delayed peak (60 min) rather than 20) is typically found in hypothalamic disease.

2. In hyperthyroidism there is no TSH response to TRH.

3. TRH test is useful in differentiating RTH from TSH omas.
    Circulating TSH shows a normal or exaggerated response to TRH that is suppressed following T3
administration ( Werner’s test: 80-100 mcg orally of T3 for 8-10 days) in patients with RTH whereas TSH secretion from autonomous tumours is unresponsive.