Thyroid-stimulating hormone (tsh), triiodothyronine (t3) and thyroxine (t4) response to intravenous and oral stimulation with synthetic thyrotropin-releasing hormone (trh) in young healthy adults

A study of the etiology of congenital hypothyroidism in the Niigata prefecture of Japan in patients born between 1989 and 2005 and evaluated at ages 5-19

BACKGROUND

The prevalence of congenital hypothyroidism (CH) increased during the period 1994-2002 in Japan. The reasons for these recently described increases in the prevalence of CH remain unclear. Moreover, the proportion of patients with different etiologies CH in the more recently diagnosed patients has not been established. In this study, we determined the etiologies of CH that were detected by neonatal screening in Niigata refecture, Japan.

METHODS

A total of 100 patients having a diagnosis of CH (41 men and 59 women, aged 5-19 years old) were evaluated. To determine the etiology of CH, the patients underwent a ¹²³I thyroidal radioiodine uptake test, a scintigram, a saliva to plasma radioiodine ratio analysis, a perchlorate discharge test, thyroid ultrasonography, measurements of thyroidal function and thyroglobulin, and a thyrotropin (TSH)-releasing hormone tolerance test.

RESULTS

Patients with overt CH (n=34, elevated TSH levels with low free thyroxine levels) made up 34% of the total group, 56% of the patients had subclinical CH (n=56, elevated TSH levels with normal free thyroxinelevels), and 10% had normal thyroid function. These were patients who were considered to have transient hypothyroidism or hyperthyrotropinemia. Thyroid dysgenesis was the diagnosis in 73% of patients with overt CH, and the most of these had ectopic thyroid tissue. In contrast, thyroid dysgenesis was the diagnosis in only 36% of the patients with subclinical CH.

CONCLUSIONS

Only 50% of our patients with CH detected by neonatal screening had thyroid dysgenesis. With an increase in the percentage of patients with subclinical hypothyroidism, the prevalence of thyroid dyshormogenesis has increased. Studies of the frequency and etiology of CH should consider overt and subclinical CH separately.

Comparative study of intravenous, nasal, oral and buccal TRH administration among healthy subjects

Four different modes of TRH application (400 micrograms iv, 1 mg nasal, 10 mg buccal and 40 mg oral) were investigated in young healthy subjects for evaluation of thyrotropin (TSH) and prolactin (PRL) stimulation. Plasma TSH, PRL, T4, T3, thyroxine-binding-globulin (TBG) were measured by radioimmunoassay. There were significant increases of TSH and PRL following TRH stimulation by all test forms. Bolus injection of TRH led to maximal TSH and PRL plasma levels within 20 min to 30 min, compared with 30 min to 45 min following nasal administration. Buccal and oral application produced more prolonged TSH and PRL increases, achieving plateau levels after 120 min to 180 min. Stimulated PRL levels were higher in women than in men. Uniformity of PRL response was better after iv or nasal than buccal and oral TRH stimulation. Known side effects were lower after nasal than iv TRH application. Buccal and oral administration provoked no side effects. Nasal TRH application seems to be a well suited test form for TSH and PRL stimulation.

Results of oral TRH test in the differentiation of compensated and decompensated autonomous thyroid nodules

In 17 patients with compensated autonomous adenomas of the thyroid, iv thyrotropin releasing hormone (TRH) tests (200 microgram) and oral TRH tests (40 mg) were performed. In nine of these patients, thyroid-stimulating hormone (TSH) 30 min after iv TRH showed a normal (greater than 2.7 microU/ml) and in eight patients a subnormal (less than 2.7 microU/ml) or negative response. However, after prolonged oral stimulation with 40 mg TRH, after 120-180 min TSH was normal (greater than 2.7 microU/ml) in 15 and subnormal (less than 2.7 microU/ml) in two patients. In 20 patients with decompensated autonomous thyroid nodules. TSH was not detectable (less than 0.8 microU/ml) after iv or oral TRH stimulation. Therefore, the oral TRH stimulation test seemed to be superior to the iv TRH test in the discrimination of compensated and decompensated autonomous adenomas of the thyroid.

Pharmacological interference with in vitro tests of thyroid function

Numerous drugs may cause changes in the serum concentrations of T4 and of T3. If such alterations are not recognized an incorrect diagnosis may result. In moderate degrees of hypo- and hyperthyroidism thyroid hormone levels may be spuriously normal, or the influence of pharmacological substances may lead to false diagnosis of thyroid disease in euthyroid patients. Since prediction of such alterations remains uncertain, it may be necessary to perform additional investigations when a potential artefact is recognized. On the other hand many pharmacological agents, especially those which interact with neurotransmitters, may influence TSH secretion, too. The TRH-test may show an increase or decreased TSH response, although complete suppression is only rarely seen during high-dose glucocorticoid treatment when low TRH doses are applied. Because of TRH-test gives such wide separation between different clinical states false interpretations are generally less likely than with drug-induced changes in T4 and T3 values.