ORIGINAL Annals of Nuclear Medicine Vol. 3, No 1, 9-13, 1989 A kinetic study on secretion and elimination of endogenous thyrotropin in the thyrotropin-releasing hormone test Ikuko TSUKAMOTO, Katashi SATOH, Kanji KOJIMA, Ichiro HINO, Yoshiro KAWASE, Hiroyuki SEO, Tsutomu MIYAMOTO, Nobuyuki HOSOKAWA, Shinsuke MATSUNO, Toyohisa TSUKAMOTO,* Kiichiro MIZUKAWA, Toyosato TAMAI, Masatada TANABE Department of Radiology and *Department of Pharmacy, Kagawa Medical School Hospital Serum thyrotropin (TSH) concentrations in normal young men were measured by a high-sensitivity immunoradiometric assay before and after intravenous administration of 500 ug of TSH-releasing hormone (TRH). A kinetic model was applied to evaluate the secretion rate both before (V0) and after (V0+V* at maximum rate) the administration of TRH, the elimination constant (K), the latent time (L) between TRH administration and start of the stimulated secretion, and the total amount of TSH (T) released in response to TRH. V0, V* and T varied widely from individual to individual, but correlated well with TSH before TRH administration (r=0.93, 0.80 and 0.87, respectively). A few minutes (1.89+-1.30 min) after the administration of TRH, the secretion of TSH (0.025+-0.016 uU/min ml) was stimulated, and the total release over about 1 h was 12.5+-5.6 uU/ml. Serum TSH was maximum at 31.5+-5.7 min. The half-time of disappearance of TSH was 42+-9 min. These data confirm that the stimulated secretion continues for more than 30 min, and that the pituitary releases 43.2+-22.9 mU of TSH (assuming the distribution volume of TSH is 5.8 % of body weight) in response to TRH, an amount which correlates closely (r=0.91) with TSH before TRH administration. Key words: Kinetic study, TSH secretion, TRH test, High sensitivity RIA INTRODUCTION MANY STUDIES on the TSH release response of the pituitary to intravenous injection of TRH have already been published. TSH, response to TRH is reported to be higher in females than in males1,2 and in females higher in the preovulatory phase than in the postovulatory phase of the menstrual cycle.2 Snyder and Utiger3,4 reported that the maximum increase in serum TSH following TRH administration increased in proportion to the TRH dose within the range 6.25-400 ug, diminished significantly with increasing age in males, and was not age-dependent in females. They also reported that there was no difference in the TSH response between males and females when their height and weight were matched. Received August 14, 1987; revision accepted May 23, 1988. For reprints contact: Masatada Tanabe D., Department of Radiology, Kagawa Medical School Hospital, 1750-1, Miki-cho, Kita-gun, Kagawa, 761-07, JAPAN. Some important parameters of TSH regulation in the TRH test remain unknown, but can be found from kinetic considerations. For example, the real amount of TSH released in response to TRH administration, which should correlate closely with the TSH secretory ability of the pituitary; the secretion and elimination rates of endogenous TSH; the latent time between TRH injection and the start of the stimulated secretion of TSH from the pituitary; the pattern of stimulated secretion of TSH, and so on. However, there are already a few kinetic studies on the TRH test. In 1986, Swartz et al.5 applied a kinetic model to the TRH test, and evaluated some parameters. They discussed the increase in TSH over the baseline, and reported that the latent time after TRH administration was 9.1+-3.7 minutes. But other reports show an increase in TSH within 5 min after TRH administration.2,3,6 In this study, we performed the TRH test on 12 normal young men. To obtain the parameters described above, we derived a theoretical expression for serum TSH by constructing a kinetic model. This model resembles that of Swartz et al.,5 but we dealt with serum TSH itself while they dealt only with the increase in TSH level over the baseline. Serum TSH before and after TRH was measured by a recently developed high-sensitivity immunoradiometric assay, and the secretion and elimination rates of endogenous TSH, the total amount of TSH released in response to TRH administration, and the latent time after TRH administration were estimated by least squares fitting. MATERIALS AND METHODS Subjects Twelve normal young men, aged 23-33 yr, volunteered for the study. None of them had a history of thyroid disease, and none had any illness or was taking any medication known or suspected to affect thyroid function. All tests were performed in the morning after an overnight fast. 500 ug of synthetic TRH (Takeda) was injected into the antecubital vein at time 0. Blood was sampled at - 10, 0 (before TRH), 3, 6, 10, 15, 20, 25, 30, 40, 50, 60, 90, 120, 160, 200 and 240 min for determination of TSH. At - 10 min, blood was also taken to determine T3 and T4. Assays Serum TSH was assayed with a commercially available immunoradiometric assay kit (TSH RIABEADS II; DAINABOT: sensitivity 0.1 uU/ml). All samples from an individual subject were analyzed in duplicate in a single assay. With respect to the kit, the intraassay variance was less than 3%, and the inter-assay variances for control serums containing 1.7 and 7.5 uU TSH/ml were 5% and 3%, respectively. T3, and T4 were measured with commercially available radioimmunoassay kits, T3 RIABEAD (DAI-NABOT; sensitivity 0.5 ng/ml) and SPAC T4 RIA kit (Daiichi; sensitivity 1.0 ug/dl), respectively. All samples were analyzed in duplicate in a single assay. The intraassay variances for the T3 and T4 assays were less than 3%. Kinetic model Administered TRH is transported to the pituitary, stimulates the synthesis and secretion of TSH,7 and is eliminated with a half life of 5.3+-0.3 min.6 To describe the variation in the TSH concentration, we constructed a simple kinetic model. This model is based on some assumptions: 1. TSH is eliminated from the blood at a rate proportional to its concentration, with an elimination constant K (min-1). 2. L minutes after the TRH injection, TSH release is stimulated from its normal rate v0 (uU/min) to a maximum rate of v0+v* (uU/ml) . 3. The stimulated rate of TSH release from the pituitary drops back to v0 at a rate proportional to its instantaneous value with the decay constant R (min-1). 4. The volume of TSH distribution V (ml) remains constant before and after TRH administration. Then the variation in the total amount of TSH distributed everywhere but in the pituitary,X (pU), is expressed as where v is the secretion rate of TSH from the pituitary, i.e., From equations (1)-(3), we get the following expressions for X. Introducing rates of release per unit volume of distribution V* (=v*/V) and V0 (=v0/V) (uU/ml min) the serum TSH concentration C (uU/ml) is given by Theoretical serum TSH concentrations were calculated from equations (6) and (7) by least squares fitting taking K, R, V*, V0 and L as five adjustable parameters. A Simplex method8 was used. The total amount of TSH released in response to TRH injection, T (uU/ml), was calculated from the following equation. The time at which C becomes the maximum concentration, Tmax, is written as All calculations were performed with a NEC PC-9801 personal computer. RESULTS AND DISCUSSION Height, weight, age and measured TSH, T3, and T4 concentrations in the 12 subjects are summarized in Table 1. The measured TSH concentrations and their SD, the simulated theoretical curve, and the increased releasing rate of TSH are shown in Fig, 1. Simulations were performed with respect to individuals and to the mean values of all subjects. The values for the determined parameters are summarized in Table 2, together with the calculated maximum TSH concentration Cmax. With respect to subjects B and K, the values for L were estimated to be less than 0. Therefore, the calculation was performed with a fixed value for L (=0). The values for R and K varied little among subjects, while the values for V0 and V* varied widely from individual to individual. The dependence of these parameters on the calculated TSH concentration was demonstrated by the data of subjects D and G in Fig, 1, which were examples of relatively high and low releasing rates, respectively. The measured TSH levels of subjects D and G were plotted together in Fig. 1. TSH concentrations before TRH injection, TSH0, correlated with Cmax, T, V0 and V* (r=0.88, 0.87, 0.93 and 0.80, respectively). V0 correlated with Cmax, T and K (r=0.76, 0.80 and 0.79, respectively). V* correlated with T (r=0.88), and Cmax correlated closely with V* and T (r=0.96 and 0.97, respectively). Dose (500 ug/weight) dependence was not observed with respect to any parameters. Considering the time required for TRH to be transported from the antecubital vein to the pituitary, and for TSH to be transported from the pituitary to the antecubital vein, the estimated value for L (1.89+-1.30 min) is reasonable, and the stimulated secretion can be considered to begin immediately after the arrival of TRH. This conclusion is different from that of a previous report. Swartz et al.5 estimated L to be 9.3 min using a kinetic model that resembles ours. They dealt only with the increase in the TSH concentration over baseline, and did not report the derivation of their expression for the TSH concentration. We consider this discrepancy as probably caused by the large intervals between samples. They collected blood samples only at 0, 15 and 30 min prior to the maximum concentration of TSH. As shown in Fig. 1, stimulated secretion of TSH started within 3 min after administration of TRH, and seemed to continue for more than 30 min. If the stimulated secretion had terminated within a few min. Cmax would not have appeared around 30 min. 1.89+-1.30 min after the administration of TRH, secretion of TSH was increased from 0.0251+-0.0156 to 0.84+-0.45 (=V0+V*) uU/ml min, and a total of 12.5+-5.6 uU/ml of TSH was released in response to TRH administration. This means that the amount of TSH released was equal to the amount normally released in 8.3 hours. TRH is supposed to stimulate both synthesis and secretion of TSH.7 Though it is not known how much TSH is newly synthesized and how much is stored in the pituitary, it seems that most of the released TSH was stored in a complete or nearly complete form, since it was released in a short time. The released TSH was eliminated with an elimination constant K (=0.0167+-0.0037 min-1); in other words, the half-time of disappearance of TSH was 42+-9 min. This value agrees well with that in another report. Odell et al.9 evaluated the secretion rate and the half-time of disappearance of TSH by administering exogenous human TSH labelled with 131-I. They reported the secretion rate of TSH (165.2+-67.4 mU/day), the volume of distribution of TSH (5.8+-2.3% of the body weight), and the half time of disappearance of TSH (53.9+-9.2 min). Though it is known that TSH concentrations show a circadian rhythm10 and a pulsatile secretion pattern,11 the variation of the TSH concentration during a day is small. Assuming the secretion rate of TSH to be constant and using the reported value for the TSH distribution volume, the secretion rate measured in our study can be rewritten as 124.9+-88.0 mU/day, which is in agreement with the study of Odell et al. and Ridway et al. (104.3+-41.4 mU/day).12 In a similar manner, the total amount of TSH released in reponse to TRH is calculated to be 43.2+-22.9 mU, which is considered to represent the secretory ability of the pituitary. As mentioned above, this value varied among subjects and correlated well with TSH0 (r=0.91) (Fig. 2). In this study, we evaluated the secretory ability of the pituitary and the elimination of TSH from a simple kinetic model without using any exogenous TSH. The agreement of the estimated values with other studies supports the validity of the kinetic model and the newly determined values. Normal serum TSH level is a value recently become measurable by immunoradiometric assay. In this study, it was 1.43+-0.72 uU/ml. Though the range of this value was very narrow compared with the range after TRH (9.14+-4.26 uU/ml at 25 min), it was found that TSH0 represents well the secretory ability of a subject. ACKNOWLEDGMENTS We wish to thank Dr. Jiro Takahara of Kagawa Medical School and Dr. Kiyoshi Yamaoka of Kyoto University for their helpful comments. Thanks are also due to Miss Yasuko Yoshida for her excellent technical assistance. REFERENCES 1. Loosen PT, Mason GA, Prange AJ: The TRH test in normal subjects: methodological considerations. Psychoneuroendocrinology 7: 147-153, 1982 2. Sanchez-Franco F, Garcia MD, Cacicedo L, et al: Influence of sex phase of the menstrual cycle on TSH response to TRH. J Clin Endocrinol Metab 37: 736-740, 1973 3. Snyder PJ, Utiger RD : Response to TRH in normal man. J Clin Endocr 34: 380-385, 1972 4. Snyder PJ, Utiger RD : Thyrotropin response to thyrotropin releasing hormone in normal females over forty. J Clin Endocrinol Metab 34: 1096-1098, 1972 5. Swartz CM, Wahby VS, Vacha R: Characterization of the pituitary response in the TRH test by kinetic modeling. Acta Endoclinol 112: 43-48, 1986 6. Bassiri RM, Utiger RD: Metabolism and excretion of exogenous thyrotropin-releasing hormone in humans. J Clin Invest 52: 1616-1619, 1973 7. Menezes-Ferreira MM, Petrick PA, Weintraub BD: Regulation of TSH bioactivity by TSH-releasing hormone and thyroid hormone. Endocrinology 118: 2125-2130, 1986 8. Nelder JA, Mead R: A simplex method for function minimization. Computer J 7: 308-313, 1965 9. Odell WD, Utiger RD, Wilbcr JF, et al : Estimation of the secretion rate of thyrotropin in man. J Clin Invest 46: 953-959, 1967 10. Vanhaelst L, Van Cauter E, Degaute JP, et al: Circadian variations of serum thyrotropin levels in man. J Clin Endocrinol Metab 35: 479-482, 1972 11. Greenspan SL, Klibanski A, Schoenfeld D, et al: Pulsatile secretion of thyrotropin in man. J Clin Endocrinol Metab 63: 661-668, 1986 12. Ridgway EC, Weintraub BD, Maloof F: Metabolic clearance and production rates of human thyrotropin. J Clin Invest 53: 895-903, 1974