Triiodothyronine and thyroxine interrelationships in health and disease.

With the recent development of radioimmunoassay techniques for the measurement of serum triiodothyronine (T3) concentration, new concepts have arisen regarding the biologic role of T3 in health and disease and its interrelationships with thyroxine (T4). An awareness of the influence of clinical conditions that affect binding of thyroid hormone to plasma proteins is required in the interpretation of moderately increased or decreased serum T3 values. Hormone preparations containing T3 may produce transient increases in T3 concentration into the hyperthyroid range. Measurements of serum T3 concentration appear to be particularly indicated in clinical situations in which hyperthyroidism is suspected but serum T3 resin uptake and serum T4 values are normal, to exclude the T3-toxicosis syndrome. Also, when serum T4 values are in the hypothyroid range, measurement of serum T3 as well as serum thyrotropin (TSH) concentrations can lead to recognition of abnormalities in thyroid gland biosynthesis. Before a diagnosis of hypothyroidism is made on the basis of a low serum T3 value, one must exclude a variety of clinical nonthyroidal conditions that can result in changes in plasma T3 protein binding or impaired peripheral conversion of T4 to metabolically active T3 without producing a hypometabolic state.     

Role of gonadal and adrenal steroids and thyroid hormones in the regulation of molting in domestic goose

Plasma levels of testosterone (T), 17-β-estradiol (E2), progesterone (P4), dehydroepiandrosterone (DHEA), corticosterone (B), thyroxine (T4) and triiodothyronine (T3) were monitored during postnuptial and the prenuptial molt in domestic goose (Anser anser domesticus) in both sexes. 1. At the beginning of postnuptial molt (when the old, worn dawny-, and cover feathers' loss starts) in ganders, the levels of T, E2, P4 decrease while DHEA and B significantly increase. The elevated levels of T4 and low T3 concentrations characteristic of the last phase of the reproduction, remain unchanged. In layers, similar changes were observed, however, B decreases. 2. In the early phase of outgrowth of wing and cover feathers, plasma levels of T, E2 and P4 are low. Elevated B, DHEA and T4 concentrations decrease in ganders, while in layers DHEA increases and B and T4 levels remain unchanged. T3 increases in both sexes. 3. The subsequent intensive outgrowth period of wing- and cover feathers both in ganders and in layers is characterized by very low levels of T, E2, DHEA and T4, but P4 increased, and T3 concentration remain high. 4. At the end of postnuptial molt - when the outgrowth of dawny, cover-, and wing feathers stops - very low T, E2, P4, DHEA and T4 levels and and high T3 plasma levels were found in both sexes. Fast increase of plasma B was detected in ganders, while in geese, B concentration remain high. 5. During prenuptial molting (outgrowth of contour and tail feathers) low E2, P4 and T4, increasing T and DHEA, but very high T3 and B plasma concentration were measured in ganders. In layers, very low T, E2, P4, DHEA and T4 levels, and very high B and T3 levels were found. 6. At the beginning of the fall-winter sexual repose (postmolting stage) T, E2, P4, DHEA and T4 levels increase, T3 and B declines in both sexes. 7. In the subsequent phase of fall-winter period (preparatory stage) there is a further increase in T, P4 and T4, a fast increase of B and a decrease of E2, DHEA and T3 in ganders. In layers, T, P4 and DHEA decrease, B increases and the T4 and T3 do not change. 8. At the beginning of reproduction high T level, unchanged DHEA, slightly declined P4, and decreased E2, T4, T3 and a strong decline of B concentrations occur in ganders. In layers, T is further increased, E2 and P4 shows high levels, and, at the same time DHEA and T3 remain unchanged, while B and T4 decrease.     

Changes in plasma testosterone, thyroxine and triiodothyronine in relation to sperm production and remex moult in domestic ganders

Changes in plasma testosterone (T), thyroxine (T4), triiodothyronine (T3), semen output and remex moult were studied in domestic ganders. A bimodal pattern in both plasma T and sperm concentration was observed during the annual cycle. Ganders started to produce semen at the end of January; maximum semen volume (0.32 +/- 0.04 ml) and sperm concentration (148 +/- 38 x 10(3)/mm3) were reached in March and a marked decrease was observed after mid-April, when the moult of the remiges began. Plasma T3 levels peaked in February (9.7 +/- 0.6 nmol.l-1) and this peak coincided with maximum T concentrations (9.8-10.4 nmol.l-1). Elevated levels of T4 were found from late February until mid-April (31.0-33.6 nmol.l-1). Plasma T concentration was low at all stages of remex moult and regrowth. Decreased T4 levels were found in ganders during remex regrowth from the "brush" to half of the full primary growth stage. Higher plasma T4 levels were found before and after this stage of the moult. A reverse pattern was observed for T3 concentrations.     

Thyroxine and triiodothyronine in milk of cows, goats, sheep, and guinea pigs

Milk was collected for the first 21 days of lactation twice daily from dairy cows and once daily from goats, sheep, and guinea pigs. Thyroxine (T4) and triiodothyronine (T3) were extracted from 100 microliter of milk using acidified ethanol. T4 and T3 were reconstituted in 100 microliter buffer and measured by radioimmunoassay. Concentrations (ng/ml) of T4 and T3 for milk of cows, goats, sheep, and guinea pigs, respectively, were: 0.97 and 0.94, 1.24 and 0.52, 0.99 and 0.79, and 1.41 and 0.53. T4 concentration for guinea pig milk was significantly higher than for cow and sheep milk, but not for goat milk (P less than 0.05). T3 was found in higher concentration in milk of cows and sheep than in milk of goats and guinea pigs (P less than 0.05). Species differences in conversion of T4 to T3 in mammary gland cells are suggested. Summations of T4 and T3 concentrations in milk indicated no differences among the four species. Regression analyses of changes in milk production, T4 and T3 concentrations, total T4 and T3 in milk per day, and ratios of T4 to T3 revealed variations in patterns. Concentrations of T4 or T3 tended to decrease as lactation progressed over 21 days. Total T3 tended to increase, and the ratio of T4 to T3 tended to decrease. Amounts of T4 and T3 available to offspring from milk were calculated to be minor sources (4 to 7%) of total requirements for maintenance of metabolic functions.     

Biological variation in Trichinella species and genotypes

At present, the genus Trichinella comprises seven species of which five have encapsulated muscle larvae (T. spiralis, T. nativa, T. britovi, T. nelsoni and T. murrelli) and two do not (T. pseudospiralis and T. papuae) plus three genotypes of non-specific status (T6, T8 and T9). The diagnostic characteristics of these species are based on biological, biochemical and genetic criteria. Of biological significance is variation observed among species and isolates in parameters such as infectivity and immunogenicity. Infectivity of Trichinella species or isolates is determined, among other considerations, by the immune status of the host in response to species- or isolate-specific antigens. Common and particular antigens determine the extent of protective responses against homologous or heterologous challenge. The kinetics of isotype, cytokine and inflammatory responses against T. spiralis infections are isolate-dependent. Trichinella spiralis and T. pseudospiralis induce different dose-dependent T-cell polarizations in the early host response, with T. spiralis initially preferentially promoting Th1-type responses before switching to Th2 and T. pseudospiralis driving Th2-type responses from the outset.     

Phylogeny and Molecular Evolution of Tetraogallus in China

In total, 535 nucleotides in the mtDNA cytochrome b have been sequenced in Tetraogallus. The 53 variable sites define 16 haplotypes in 53 Tetraogallus himalayensis samples, 1 haplotype in 2 T. altaicus samples, and 6 haplotypes in 19 T. tibetanus samples. The lowest genetic distance is between T. himalayensis and T. altaicus (0.011). The divergent time is 0.69 Ma BP between T. himalayensis and T. altaicus, 4.06 Ma BP between T. himalayensis and T. tibetanus, and 4.19 Ma BP between T. altaicus and T. tibetanus. The evidence of this work showed that T. altaicus should fall under the dark-bellied group of Tetraogallus. The dark-bellied and white-bellied speciation first occurred along with the uplift of the Qinghai-Tibet Plateau. A maximum glacier (0.80-0.60 Ma BP) led to the formation of T. altaicus.     

Effects of hypo- and hyper-thyroidism on in vivo lipogenesis in fed and fasted rats

Thyroidectomized rats (T) daily injected with either 0, 0.1, 1.8 or 25 microgram of L-thyroxine/100 g body wt. were compared with intact controls (C). The appearance of radioactivity in fatty acids 30 min after the i.p. injection of (3-14C)pyruvate was reduced in adipose tissue and enhanced in liver of T+25, being no differences between the other groups and C. (14C)-Fatty acids are reduced with 3 h of fasting only in the adipose tissue of T+1.8 and C, while 24 h produces a reduction in liver in the T+1.8, T+25 and C, and in adipose tissue in the T+1.8 and C animals. The highest percentage of radioactivity was observed in the liver glyceride glycerol fraction, being greater in T+25 than in the other groups. Fasting produces an increment in the (14C)-glyceride glycerol fraction. Being significant only in thehypothyroid animals in both liver and adipose tissue. The most sensitive parameter to fasting was the formation of (14C)-non-saponifiable lipid in both the C and T+1.8 animals, while it does not change in T+0 or T+0.1, but is enhanced within 24 h in the adipose tissue of T+25. It is proposed that most of the observed changes are due to the other endocrine disfunction s which appear in hypo- and hyperthyroidism, as the in vivo results do not comply with in vitro effects of thyroxine onlipogenesis of others.     

Differing roles of inflammation and antigen in T cell proliferation and memory generation

Recent studies have demonstrated that viral and bacterial infections can induce dramatic in vivo expansion of Ag-specific T lymphocytes. Although presentation of Ag is critical for activation of naive T cells, it is less clear how dependent subsequent in vivo T cell proliferation and memory generation are upon Ag. We investigated T cell expansion and memory generation in mice infected alternately with strains of Listeria monocytogenes that contained or lacked an immunodominant, MHC class I-restricted T cell epitope. We found substantial differences in the responses of effector and memory T cells to inflammatory stimuli. Although effector T cells undergo in vivo expansion in response to bacterial infection in the absence of Ag, memory T cells show no evidence for such bystander activation. However, Ag-independent expansion of effector T cells does not result in increased memory T cell frequencies, indicating that Ag presentation is critical for effective memory T cell generation. Early reinfection of mice with L. monocytogenes before the maximal primary T cell response induces typical memory expansion, suggesting that the capacity for a memory T cell response exists within the primary effector population. Our findings demonstrate that T cell effector proliferation and memory generation are temporally overlapping processes with differing requirements for Ag.     

Membrane-bound and free polysomes in transformed and untransformed fibroblast cells

A rapid and quantitative fractionation procedure has been used to measure the amounts of free and membrane-bound polysomes in growing and stationary Py3T3 and 3T3 (mouse) cells. A comparison of growing 3T3 and Py3T3 cells does not reveal any significant differences with regard to the ratio of the two polysome fractions. The amount of free and membrane-bound polysomes decreases in both 3T3 and Py3T3 cells as they approach the stationary state, an effect which is much more pronounced for free polysomes. At greatly reduced growth rates or in stationary cells, however, the amount of membrane-bound polysomes doubles in Py3T3 cells while it decreases even further in 3T3 cells. By contrast, the amount of free polysomes remains at a reduced level in both 3T3 and Py3T3 cells when cell multiplication is inhibited.Based on the hypothesis that membrane proteins are selectively synthesized on membrane-bound polysomes, an attempt is made to relate the results to accumulated data in the literature and discuss its possible significance with respect to the loss of growth control in Py3T3 cells.     

Nuclear thyroxine and triiodothyronine binding in mononuclear cells in dependence of age

Nuclear binding of thyroxine (T4) and triiodothyronine (T3) in mononuclear blood cells was investigated in 12 young (age 16-30 years) healthy subjects (group A), in 12 middle-aged (age 31-60 years) healthy subjects (group B) and in 12 elderly (61-90 years) healthy subjects. Serum free T3 was depressed in group C as compared to the younger age groups, whereas serum free T4 and TSH did not differ between the groups. Maximal specific nuclear binding capacity for both T4 and T3 decreased with increasing age, T4 group A: 1.2 fmol T4/100 micrograms DNA, group B: 1.2 fmol T4/100 micrograms DNA, group C: 0.7 fmol T4/100 micrograms DNA; T3 group A: 1.7 fmol T3/100 micrograms DNA, group B: 1.0 fmol T3/100 micrograms DNA, group C: 0.9 fmol T3/100 micrograms DNA. The equilibrium association constant (Ka) for T4 increased with age, group A: Ka = 3.3 X 10(9) l/mol, group B: Ka = 3.2 X 10(9) l/mol, group C: Ka = 6.4 X 10(9) l/mol, whereas Ka for nuclear binding of T3 decreased with age group A: Ka = 3.9 X 10(9) l/mol, group B: Ka = 5.9 X 10(9) l/mol, group C: Ka = 1.8 X 10(9) l/mol. We conclude that, whereas the opposite variations of nuclear capacity and binding affinity for T4 tend to preserve the nuclear T4 concentration, the nuclear T3 concentration definitely decreases with age. The unaltered serum levels of TSH suggest that the decrease of both serum levels of free T3 and the nuclear T3 concentration might represent physiologically changes in old age.     

Circadian variations in concentrations of plasma thyroxine and triiodothyronine in man.

The plasma concentrations of thyroxine (T4), triiodothyronine (T3), and total protein (Pr) were measured at 2-h intervals in 8 male subjects during two 24-h periods. Plasma T4 and T3 levels varied significantly during the day. T4 values were highest at 0900 hours and thereafter declined rapidly reaching lowest levels at 1500-1700 hours (mean decrement, 13.2% of 0.00-hour value). Plasma T3 was highest at 0900 hours and lowest at 1700-1900 hours (mean decrement, 16.7% of 0900-hour value). Fluctuations observed in Pr were not significant. Variations in plasma T4 and T3 appeared concordant with respect to time, since no significant variation was detected in T3:T4 plasma concentration ratios. In view of previous studies that have demonstrated circadian variations in the binding of thyroid hormones by plasma proteins, it is suggested that the observed temporal variations in plasma concentrations of T3 and T4 reflect parallel changes in the capacity or affinity of specific plasma binding proteins of these iodothyronines.     

Changes in thyroid hormone levels in chicken liver during fasting and refeeding

In chickens, fasting results in increased plasma thyroxine (T(4)) levels and decreased plasma 3,5,3'-triiodothyronine (T(3)) levels. Refeeding, in turn, restores normal plasma T(3) and T(4) levels. The liver is an important tissue for the regulation of circulating thyroid hormone levels. Previous studies demonstrated that the increase in hepatic type III deiodinase in fasted chickens plays a role in the decrease of plasma T(3). Another factor that could be important is the level of T(4) and T(3) uptake by the liver. In mammals, caloric restriction is known to diminish transport of T(4) and T(3) into tissues. The present study examines whether this is also the case in chicken. Four-week-old chickens were subjected to a 24-h starvation period followed by refeeding. Blood and liver samples were collected at the start of refeeding and at different times of refeeding. Thyroid hormone levels were measured directly in plasma and in tissues following extraction. The results demonstrate that intrahepatic T(4) levels are increased and T(3) levels are decreased in fasted compared to ad libitum fed chickens. The parallel changes in plasma and hepatic T(3) and T(4) content demonstrate that T(4) availability in liver tissue is not diminished during fasting, suggesting that in chicken thyroid hormone uptake by the liver is not affected by nutritional status.     

Activated T cells in vivo and in vitro: divergence in expression of Tac and Ia antigens in the nonblastoid small T cells of inflammation and normal T cells activated in vitro

Elevated numbers of non-blastoid T cells expressing either the Tac or Ia antigens were found on separate cell populations in inflammatory synovial tissues and fluids of individuals with arthritis. Those synovial T cell preparations containing Tac+ cells exhibited marked proliferation upon the addition of IL 2 without concomitant mitogen stimulation; T cell eluates containing Ia+ but not Tac+ T cells did not show significantly increased levels of blastogenesis. Paired T cell preparations from blood had only minor increases in the number of Tac+ T cells and moderate increases in the number of Ia+ cells. The blood cells did not exhibit significant proliferation to IL 2. In contrast mitogen or allogeneic activation of T cells induced blastoid cells that expressed abundant per cell amount of Ia or Tac antigens. These blastoid cells resembled the small T cells of inflammation in having only very limited overlap between the population that bore Ia antigens and those with the Tac antigen; however, there was a preponderance of Tac-bearing cells.     

13C NMR relaxation times of hepatic glycogen in vitro and in vivo

The field dependence of relaxation times of the C-1 carbon of glycogen was studied in vitro by natural-abundance 13C NMR. T1 is strongly field dependent, while T2 does not change significantly with magnetic field. T1 and T2 were also measured for rat hepatic glycogen enriched with [1-13C]glucose in vivo at 4.7 T, and similar relaxation times were observed as those obtained in vitro at the same field. The in vitro values of T1 were 65 +/- 5 ms at 2.1 T, 142 +/- 10 ms at 4.7 T, and 300 +/- 10 ms at 8.4 T, while T2 values were 6.7 +/- 1 ms at 2.1 T, 9.4 +/- 1 ms at 4.7 T, and 9.5 +/- 1 ms at 8.4 T. Calculations based on the rigid-rotor nearest-neighbor model give qualitatively good agreement with the T1 field dependence with a best-fit correlation time of 6.4 X 10(-9) s, which is significantly smaller than tau M, the estimated overall correlation time for the glycogen molecule (ca. 10(-5) s). A more accurate fit of T1 data using a modified Lipari and Szabo approach indicates that internal fast motions dominate the T1 relaxation in glycogen. On the other hand, the T2 relaxation is dominated by the overall correlation time tau M while the internal motions are almost but not completely unrestricted.     

Thyrotropic and peripheral activities of thyrotrophin and thyrotrophin-releasing hormone in the chick embryo and adult chicken

The influence of an intravenous injection of thyrotrophin-releasing hormone (TRH) and bovine thyrotrophin (TSH) on circulating levels of thyroid hormones and the liver 5'-monodeiodination (5'-D) activity is studied in the chick embryo and the adult chicken. In the 18-day-old chick embryo, an injection of 1 microgram TRH and 0.01 I.U. TSH increase plasma concentrations of triiodothyronine (T3) and of thyroxine (T4). TRH, however, preferentially raises plasma levels of T3, resulting in an increased T3 to T4 ratio, whereas TSH preferentially increases T4, resulting in a decreased T3 to T4 ratio. The 5'-D-activity is also stimulated following TRH but not following TSH administration. The increase of reverse T3 (rT3) is much more pronounced following the administration of TSH. In adult chicken an injection of up to 20 micrograms of TRH never increased plasma concentrations of T4, but increases T3 at every dose used together with 5'-D at the 20 micrograms dose. TSH on the other hand never increased T3 or 5'-D, but elevates T4 consistently. It is concluded that TSH is mainly thyrotropic in the chick embryo or adult chicken whereas TRH is responsible for the peripheral conversion of T4 into T3 by stimulating the 5'-D-activity. The involvement of a TRH induced GH release in this peripheral activity is discussed.     

Increased T gamma and T mu cells in BALB/c mice with IgG and IgM plasmacytomas and hybridomas

The results of previous studies in our laboratory have shown that mice bearing plasmacytomas and hybridomas that secrete IgA or IgE are accompanied by increased frequencies of Lyt-1-2+ T lymphocytes bearing Fc receptors (FcR) for IgA (T alpha) or IgE (T epsilon), respectively. The present study was undertaken to examine whether IgG- or IgM-secreting tumors influenced the frequency of T lymphocytes that express FcR for IgG or IgM. We studied mice bearing IgG- and IgM-secreting plasmacytomas and hybridomas. BALB/c mice injected subcutaneously with the IgG-secreting hybridoma HDP1 (gamma 1 kappa, anti-TNP) were sequentially examined for the frequencies and Lyt phenotypes of splenic lymphocytes bearing FcR for IgG (T gamma), IgM (T mu), and IgA (T alpha). A threefold increase in the frequency of T gamma lymphocytes that were Lyt-1-2+, L3T4- was seen. The frequencies of T mu and T alpha lymphocytes in these mice were not significantly altered. Similarly, mice injected subcutaneously with the IgM-secreting plasmacytoma MOPC 104E (mu lambda, anti-dextran) or the IgM-secreting hybridoma C1D1 (mu kappa, anti-ox RBC) were examined sequentially for the frequencies of T gamma, T mu, and T alpha lymphocytes. Mice with established IgM subcutaneous tumors showed a twofold increase in splenic, nylon wool-nonadherent T mu lymphocytes. This was associated with a relative increase in Lyt-2+ splenic T lymphocytes and a relative decrease in Lyt-1+ splenic T lymphocytes. No changes were observed in the frequencies of either T gamma or T alpha lymphocytes. These studies extend to IgG and IgM the observation that plasmacytomas and hybridomas secreting immunoglobulins of a specific isotype cause an expansion of T lymphocytes bearing FcR specific for the corresponding isotype. The expansion of FcR+ Lyt-1-2+ T lymphocytes likely represents an exaggerated, but otherwise normal, immunoregulatory response of the host. These cells may be an important element in the regulation of isotype expression.     

Amiodarone inhibits T4 to T3 conversion and alpha-glycerophosphate dehydrogenase and malic enzyme levels in rat liver

Amiodarone has been found to decrease serum T3 by blocking peripheral T4 5'-deiodinase. This reduction in T3 levels may contribute to the effectiveness of this drug in moderating cardiac arrhythmias. To further characterize the effect of amiodarone on thyroid hormone metabolism and biological action, male Sprague-Dawley rats were thyroidectomized and then fed 500 ug T4 or 50 ug T3 and 500 mg amiodarone/kg of powdered diet for 6 to 8 weeks. Hepatic and cardiac levels of T4, T3, alpha-glycerophosphate dehydrogenase (GPD) and malic enzyme (ME) were used as indicators of thyroid hormone availability and action at the cellular level. Conversion of T4 to T3 was measured in liver homogenates. Serum TSH, T4 and T3 were also measured. Amiodarone reduced hepatic GPD and ME in thyroidectomized rats receiving dietary T4. Liver T4 levels were significantly increased by amiodarone and the T3/T4 ratio was reduced (P less than .05). Amiodarone inhibited hepatic T4 to T3 conversion and decreased serum T3. The decreased T3 action at the cellular level, indicated by the reduction in hepatic GPD and ME, is not due to pharmacologic effects of amiodarone since these enzyme levels were not affected by amiodarone in thyroidectomized rats replaced with T3.     

Comparison between the thyrotropin response to thyrotropin-releasing hormone in summer and that in winter in normal subjects.

A comparison was made between the thyrotropin (TSH) response to 500 microgram thyrotropin-releasing hormone (TRH) in summer and that in winter in ten healthy normal adults living in Supporo. The serum resin triiodothyronine (T3) uptake (RT3U), thyroxine (T4) and T3 levels were also measured. While the TSH response to TRH in summer was similar to that in winter, serum T3 concentration and free T3 index were significantly higher in winter than in summer, associated with the similar values in RT3U and T4 levels in serum. Independently measured 86 specimens (43 in summer and 43 in winter) from normal adults living in the same district also showed a significant increase in serum free T3 index as well as a slight elevation of serum T3 concentration in winter but not in serum T4 level. These results indicate that the primary change in cold winter would be the stimulation of peripheral conversion of T4 to T3 rather than the activation of hypothalamo-pituitary-thyroid axis. The relevance of this interpretation was discussed.     

Mutational analysis of polyomavirus small-T-antigen functions in productive infection and in transformation.

The function of polyomavirus small T antigen in productive infection and in transformation was studied. Transfection of permissive mouse cells with mixtures of mutants that express only one type of T antigen showed that small T antigen increased large-T-antigen-dependent viral DNA synthesis approximately 10-fold. Under the same conditions, small T antigen was also essential for the formation of infectious virus particles. To analyze these activities of small T antigen, mutants producing protein with single amino acid replacements were constructed. Two mutants, bc1073 and bc1075, were characterized. Although both mutations led to the substitution of amino acid residues of more than one T antigen, the phenotype of both mutants was associated with alterations of the small T antigen. Both mutant proteins had lost their activity in the maturation of infectious virus particles. The bc1075 but not the bc1073 small T antigen had also lost its ability to stimulate viral DNA synthesis in mouse 3T6 cells. Finally, both mutants retained a third activity of small T antigen: to confer on rat cells also expressing middle T antigen the ability to grow efficiently in semisolid medium. The phenotypes of the mutants in these three assays suggest that small T antigen has at least three separate functions.     

Existence of extrathyroidal conversion inhibitor (IEC) in starved animals and its influence on thyroid hormones deiodination in liver and kidney in vitro

To find out whether an inhibitor of extrathyroidal conversion of iodothyronines is present in sera of starved animals, pig liver and kidney homogenates were incubated with T4, T3 or rT3 and dithiotreitol in the presence of evaporated diethyl ether extracts of sera obtained from fed and starved (1-12 days) rabbits. Sera extracts of short-term (1-4 days) starved rabbits caused a significant inhibition of T4 to T3 conversion (54% on day 3) and T4 to rT3 deiodination (52% on day 2) in liver homogenates. Extracts of sera from long-term (8 and 12 days) starved animals diminished only liver T4 to T3 conversion on day 8 and had no influence on liver T4 to rT3 conversion. 5'-deiodination of rT3 (to 3,3'-T2) in liver was gradually decreased by extracts of sera from animals starved during 2-12 days. Liver rT3-5-deiodination (to 3',5'-T2) was significantly impaired on day 4 and totally depressed by long-term starvation. In vitro T3 to 3,3'-T2 conversion in liver was markedly (59-103%) increased by ether extracts of sera from short-term fasted rabbits and considerably inhibited (62-72%) by long-term fasting. T4 to T3 conversion in kidney was significantly influenced by sera extracts obtained neither from short-term fasted rabbits and considerably inhibited (62-72%) by long-term fasting. T4 to T3 conversion in kidney was significantly influenced by sera extracts obtained neither from short-term nor from long-term fasted rabbits but T4-5-deiodination (to rT3) was reduced by sera extracts of short-term fasted animals.(ABSTRACT TRUNCATED AT 250 WORDS)