Simultaneous measurement of thyroxine and triiodothyronine peripheral turnover kinetics in man

Serum triiodothyronine (T(3)) kinetics in man have been difficult to define presumably due to the interference of iodoproteins generated during the peripheral metabolism of T(3). The use, in the present study, of an anion-column chromatographic method for separation of serum T(3) as well as thyroxine (T(4)) from these iodoproteins has overcome this technical handicap. Simultaneous measurement of serum (125)I-T(3) and (131)I-T(4) kinetics were performed in 31 subjects from the clinical categories of euthyroid, primary hypothyroid, thyrotoxic and posttreatment hypothyroid Graves' disease, factitial thyrotoxic, and idiopathically high and low thyroxinebinding globulin states. The normal mean T(3) fractional turnover rate (kT(3)) was 0.68 (half-life = 1.0 days), increased in toxic Graves' disease patients to 1.10 (half-life = 0.63 days), and decreased in primary hypothyroid patients to 0.50 (half-life = 1.38 days). The mean T(3) equilibration time averaged 22 hr except in hypothyroid and high thyroxine-binding globulin (TBG) patients where the equilibration period was delayed by 10 hr. The mean T(3) distribution space in normal subjects was 38.4 liters. This was reduced in subjects with high TBG levels (26 liters) and increased in patients with low TBG and in all hyperthyroid states (53-55 liters). The normal serum T(3) concentration was estimated by radioimmunoassay to be 0.106 mug/100 ml. Combined with the mean T(3) clearance value of 26.1 liters/day, the calculated T(3) production rate was 27.6 mug/day. The mean T(3) production rate increased to 201 mug/day in thyrotoxic Graves' disease patients and was reduced to 7.6 mug/day in primary hypothyroid subjects. T(3) production rate was normal in subjects with altered TBG states. The ratio of T(3) to T(4) production rate in normal subjects was 0.31 and was unchanged in patients with altered TBG values. This ratio was increased in all Graves' disease patients with the highest value being 0.81 in the posttreatment hypothyroid Graves' disease group. This apparent preferential production of T(3) may have been responsible for the retention of rapid turnover kinetics for T(3) and T(4) observed in treated Graves' disease patients. The finding that factitial thyrotoxic patients also displayed similar rapid T(3) and T(4) turnover kinetics indicates that these alterations are not a unique feature of Graves' disease per se. When comparing the peripheral turnover values for T(3) and T(4) in man, it is apparent that alterations in metabolic status and serum TBG concentration influence both hormones in a parallel manner; however, changes in metabolic status seem to have a greater influence on T(3) kinetics while alterations in TBG concentrations have a greater effect on T(4). These observations probably relate to the differences in TBG binding affinity and peripheral tissue distribution of these two hormones.

The role of glucocorticoids in the regulation of thyroid function in man

The diurnal variation in thyroidal iodine release previously observed in euthyroid subjects appears to correlate with variations in serum immunoassayable thyrotropin (TSH). The hypothesis is advanced that this diurnal rhythm seems to be primarily regulated by a negative feedback action of circulating hydrocortisone. The administration of maintenance doses of hydrocortisone to patients with primary adrenal insufficiency and pharmacological doses to euthyroid subjects was accompanied by an acute suppression in both thyroidal iodine release and serum TSH values. An escape from glucocorticoid suppression was observed to occur in 2 or 3 days with the resumption of a near-normal thyroidal iodine release rate but was accompanied by a dampening or absence of the normal diurnal rhythm. Withdrawal of pharmacological doses of glucocorticoids in euthyroid subjects and maintenance doses in primary hypoadrenal patients was accompanied by transient stimulation of both serum TSH and thyroidal iodine release values. The study of a patient before and after cryohypophysectomy indicated that the rebound response in thyroid release after steroid withdrawal may be a useful testing procedure to indirectly assess the hypothalamicpituitary reserve capacity of TSH.

Abnormalities in thyroid hormone production and metabolism in hyperthyroidism

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Influence of immunization procedures on the titer, affinity and specificity of antisera to glycopolypeptides

HLH, HTSH, HCG and HFSH are glycopolypeptides with similar molecular weights and immunological properties. Production of antisera useful in specific radioimmunoassays has been difficult. Immunization of rabbits with each of these hormones homogenized in complete Freund's adjuvant is routinely followed by antiserum production. Titers rise progressively with repeated immunizations for ten weeks or more and then the rate of increase falls. Rabbits producing higher titers of antiserum after two immunizations generally continue to produce higher titers during additional immunizations, thus selection of animals early during immunization is possible. Affinity of the antisera against all four hormones generally also increases during the course of immunization. Several ways of estimating affinity of antisera are presented. Specificity of antisera is variable, particularly against HFSH. Review of published techniques and our own experience reveals that all specific anti HFSH has been obtained after two to four immunizations; those after five or more immunizations have shown complete cross reaction with HLH, HTSH, and HFSH. These observations may explain the failure of some investigators to obtain specific antisera. For HFSH antisera, affinity progressively increases, but specificity appears to decrease with frequency of immunization. For HTSH and HCG antisera, specificity is retained or increases with repeated immunizations, and specific high affinity antisera are more regularly produced. Antisera capable of use to produce specific radioimmunoassays need not necessarily contain only antibodies against the hormone to be measured. Thus antisera against LH might be present in an antiserum used to quantify HFSH, if the anti HLH did not bind HFSH in either radioactive or nonradioactive form. Specificities of antisera vary a great deal and consideration of the nature of cross reacting substances would be effective in producing specificity. Thus when HLH reacted completely and equally with HFSH in an HFSH assay, absorbtion would completely neutralize all anti HFSH antibodies. If HLH and HFSH had dose response curves with quite different slopes (and hence different affinities for the anti HFSH), absorbtion is possible. When two or more populations of antibodies against HFSH are demonstrable by the presence of biphasic dose response curves in the assay, and if one population is specific, absorbtion of the nonspecific population may be possible.

Acute release of thyrotropin in the newborn

Measurements of serum thyrotropin (TSH) concentrations were conducted in maternal and fetal blood during labor and delivery and the early postnatal and neonatal periods. Mean TSH concentration was significantly higher in cord blood (9.5 muU/ml) than maternal blood (3.9 muU/ml), a finding suggesting a fetal-maternal TSH gradient at term. Serum TSH concentration in the newborn increased rapidly to mean levels of 60 muU/ml at 10 min and 86 muU/ml at 30 min of age. Between 30 min and 3-4 hr serum levels decreased rapidly, then fell more gradually to a mean concentration of 13 muU/ml at 48 hr. The half-time of the decrease in serum TSH concentration between 30 and 90 min was 77 min, a value slightly greater than the half-time of disappearance of radioiodinated TSH measured in adults. This indicates that the early high rate of TSH secretion in the newborn ceases by 30 min, and that the rapid rise and fall in serum TSH concentrations may represent release of stored pituitary TSH. A more chronic TSH hypersecretion persisted throughout the first 24-48 hr of extrauterine life. Measurements of serum PBI concentrations were conducted in a separate group of maternal and cord blood samples and in the newborn infants during the first 48 hr of extrauterine life to relate the TSH and serum hormonal iodine concentration changes. Serum protein-bound iodine (PBI) concentrations were similar in maternal and cord blood, increased significantly by 4 hr of age in the newborn, and peaked at about 24 hr, presumably in response to the TSH hypersecretion. The pattern of TSH hypersecretion was similar in infants delivered vaginally and by caesarean section. Maternal serum TSH concentrations were stable throughout the perinatal period. Warming the infants at 99-103 degrees F during the first 3 hr of life did not prevent the early, acute release of thyrotropin. Cooling of warm infants at room temperature (72-78 degrees F) between 3 and 4 hr resulted in a decrease in mean rectal temperature of 3.3 degrees F and produced a significant increment in serum TSH concentration. These data suggest that the mechanism of the early, acute release of thyrotropin in the newborn may involve a potent stimulus other than cooling. However, the increase in serum TSH stimulated by delayed (3-4 hr) cooling indicates that neonatal hyperthyroxinemia is, at least, augmented by extrauterine cooling. Thus, cold exposure is capable of increasing TSH secretion in humans.

Norepinephrine inhibition of vasopressin antidiuresis

The effect of norepinephrine on exogenous vasopressin antidiuresis was investigated in water-loaded subjects. After an initial 2 to 3 hr period of water loading (phase 1), 10-100 mU of vasopressin per hr were infused at a constant rate for 1 hr (phase 2) followed by infusion of 10-100 mU of vasopressin per hr plus 600 mug of l-norepinephrine per hr for 1 hr (phase 3). Endogenous creatinine clearance, osmolal clearance, and free water clearance (in milliliters/minute) and sodium and chloride excretion (in milliequivalents/minute) were measured. In 10 subjects given 10-20 mU of vasopressin per hr during phases 2 and 3, free water clearance decreased significantly from phase 1 to phase 2 (9.3 to 0.15, P = 0.001) and increased during phase 3 norepinephrine infusion to 4.7 ml/min (P = 0.001). A comparable decrease in phase 2 free water clearance was observed in four subjects given 50 or 100 mU of vasopressin per hr during phases 2 and 3 (P < 0.01); however, the phase 3 norepinephrine infusion in these subjects was not associated with an increase in free water clearance. Creatinine clearance, osmolal clearance, and sodium and chloride excretion were unchanged throughout the studies in both groups of subjects.A two phase study in seven subjects confirmed that 10, 20, or 75 mU of vasopressin per hr susstained antidiuresis during phase 2 for at least 2 hr and that free water clearance values were essentially constant in the individual subject after the first 30 min of infusion. The magnitude of the (phase 3) norepinephrine-induced increase in free water clearance (4.5 +/- 0.64 ml/min) during infusion of 10-20 mU of vasopressin per hr, the failure of norepinephrine to increase free water clearance during infusion of 50-100 mU of vasopressin per hr, and the relatively constant endogenous creatinine and osmolal clearance rates would suggest that the norepinephrine inhibition of vasopressin antidiuresis was not the result of alterations in renal blood flow. A post-phase 3 infusion of vasopressin in four subjects resulted in a marked decrease in free water clearance, indicating that the norepinephrine inhibition of vasopressin antidiuresis was not accountable on the basis of decreased medullary hypertonicity. These data support the hypothesis that catecholamine blocks the cellular mechanism of vasopressin antidiuresis in vivo. The observation that norepinephrine did not inhibit the antidiuresis produced by the infusion of 50 or 100 mU of vasopressin per hr suggests that this inhibition might be competitive. A possible role of catecholamine in the mechanism of cold diuresis is suggested.