The standard clinical dose administered as an intravenous bolus injection is 200-500┬╡g. Transient mild nausea, a sense of urinary urgency, and moderate (rarely marked) increases in blood pressure occur as side effects of injection. The surge of TSH release induced by TRH injection leads to a detectable rise in plasma triiodothyronine (T3) and also an increase in thyroxine (T4) release, although the latter is usually not large enough to be reflected in a significant increase in circulating levels of T4.
A striking feature of the TRH-induced TSH response is that its effects are blocked by prior treatment with thyroid hormone. Indeed, the interaction of the negative feedback effects of thyroid hormone on the pituitary with the stimulating effects of TRH is a major element in the feedback control of TSH secretion (Fig. 1.13).
