A new method for the measurement of thyroidal iodine release in man

Abstract

ABSTRA CT A new method is described for the detection of variations in the release rate of all iodinated products by the thyroid gland in man. 'I was employed to endogenously label the thyroid gland, and a parenterally administered '81I thyroxine dose was used to generate a 181I reference source. Thyroidal iodine release activity was quantitatively assessed by the measurement of variations in 'I/I ratio values obtained in timed urine samples. Diurnal variation in thyroid release patterns was observed in euthyroid subjects which was promptly suppressed by exogenous triiodothyronine administration and was simulated by the intramuscular injection of 0.25-0.50 U of bovine thyrotropin. The zenith value occurred at 4:00 a.m. ±3.4 hr (SD) and the nadir at 5:00 p.m. ±3.6 hr. The absence of a diurnal pattern in patients with thyrotoxicosis and in secondary hypothyroidism indicated that this diurnal fluctuation was under thyrotropin (TSH) regulation. This new method also promises to be a useful tool for the study of the intrathyroidal recycling of iodide from the iodotyrosine pool and the detection of factors which may acutely alter thyroid function.

Alterations in thyroid iodine release and the peripheral metabolism of thyroxine during acute falciparum malaria in man

“…6) clearly confirms the marked, although transient, depression in thyroidal release occurring during acute illness. This phenomenon has been observed previously in one patient during stress and infection by Nicoloff (11). This method of assessing thyroidal iodine release depends upon generation of 'I from peripheral 1'I-T4 at a relatively constant rate, so that changes in the UTI: U'I ratio reflect alterations in endogenous 'I secretion.…”

“…After the administration of 'I to label thyroidal iodine, and 'I-T4 to label the extrathyroidal thyroxine pool, values for the ratio of 'I: 'I in urine pass through equilibratory and transition phases before assuming a progressively rising slope, said to represent 'I release from the thyroid (11). In the steady state, urinary radioiodine derived from peripheral 'I-T& changes at a relatively constant rate, and any changes in the slope of U'I: U'=I thus reflect alterations in thyroidal iodine release, which are in turn mediated via changes in TSH secretion (20).…”

“…There are a number of studies which suggest by indirect methods an inhibition of thyrotrophin (TSH) release by fever or stress (11,26,47). Kohler, O'Malley, Rayford, Lipsett, and Odell reported a suppression of immunoassayable TSH in three hypothyroid subjects after administration of pseudomonas pyrogen (Piromen) (48).…”

“…In the assessment of thyroidal iodine release, the method offers several advantages over the serial epithyroid counting technique, in view of interpretive and technical shortcomings inherent in the latter method. Another very similar method for estimating thyroidal iodine release (11) was also employed. To our knowledge, this is the first time these methods have been used in man to examine the sequential changes in thyroid hormone metabolism that occur during infection.…”

Assessment of Thyrotropin-Releasing Hormone and Thyrotropin Reserve in Man

“…This method has been described in detail in previous reports (17,18 measurements so that 10-15 AiCi would be delivered to the thyroid gland. 50 AGCi of ['Ithyroxine (T4) (specific activity was greater than 40 ,Ci/ig at time of administration) was then given intravenously to serve as a reference isotope.…”