The Triiodothyronine and Thyroxine Response to Thyrotrophin‐releasing Hormone in the Assessment of the Pituitary‐thyroid Axis

Abstract

SUMMARY The thyroidal component of the response to thyrotrophin releasing hormone (TRH) has been studied by measurement of serum triiodothyronine (T3) and thyroxine (T4). In normal subjects a significant increase in serum T3 followed intravenous administration of TRH 200 μg. A less consistent increase was detected in serum T4 concentration. Oral TRH at a dose of 40 mg caused a significant rise in serum T4 in normal subjects. The increase in T4 at 24 hr was considered to provide a convenient index of pituitary and thyroidal response. Fourteen patients with pituitary disease, of whom twelve were euthyroid, were investigated by an oral TRH test. The majority of these patients retain a normal pituitary TSH reserve though a small group can be identified in whom it is diminished. A significant correlation was obtained between the T4 and TSH response to oral TRH. It is suggested that measurement of serum concentration of thyroid hormone is of value in the assessment of the pituitary response to TRH and the integrity of the pituitary‐thyroid axis.

Combined test of anterior pituitary function in children.

“…However, for suspected hypothalamic or pituitary disease, the test is valuable and its combined administration with other releasing hormones or insulin has proved satisfactory. In adults the release of TSH following TRH is accompanied by a peak increase of serum triiodothyronine at 2-3 hours and serum thyroxine at 6-8 hours (Shenkman et al, 1972;Lawton et al, 1973;Patel and Burger, 1973). Although our study confirms the proportionately greater rise of triiodothyronine compared with thyroxine at 2 hours, 3 children showed no rise above basal levels of either.…”

An Electroencephalographic Investigation of Short‐term Effects of Three Hypothalamic Hormones (Trh, Lh/Fsh‐rh, Gh‐rih) in Normal Subjects

“…It is possible that changes in serum levels of T3 (triiodothyronine) or T4 (thyroxine) caused by TRH could affect the CNV. However, according to Lawton, Ellis & Sufi (1973), T3 levels start to rise at about 30 min and do not reach a peak until 2-4 h after intravenous TRH, and T4 is variable, not always rising after 3 h. The effects of such long term influences would not have been detected in the present experiments.…”

Bromocriptine Suppression of the Thyrotrophin Response to Thyrotrophin Releasing Hormone

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A single oral dose of 5 mg of bromocriptine significantly lowered the TSH response to 200 pg TRH intravenously in eight healthy men compared with control experiments in the same subjects. This finding may be relevant in chronic bromocriptine therapy.Bromocriptine has previously been shown t o lower prolactin concentrations in hyperprolactinaemia (Besser et al, 1972), growth hormone concentrations in acromegaly (Liuzzi et al, 1974), corticotrophin concentrations in Cushing's disease and Nelson's syndrome (Benker et al, 1976), and luteinizing hormone release in women (Lachelin et al, 1977). More recently, a single dose of bromocriptine has been shown to decrease the raised TSH seen in primary hypothyroidism (Miyai et al, 1974).

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