Thyroid stimulating hormone upregulates secretion of cathepsin B from thyroid epithelial cells

Constant levels of thyroid hormones in the blood are principal requirements for normal vertebrate development. Their release depends on the regulated proteolysis of thyroglobulin which is extracellularly stored in the follicle lumen under resting conditions. Thyroglobulin is proteolytically degraded to a major part in lysosomes, but in part also extracellularly leading to the release of thyroxine. Extracellularly occurring lysosomal enzymes are most probably involved in the proteolytic release of thyroxine. In this study we have analyzed the secretion of cathepsin B by thyroid follicle cells (primary cells as well as FRTL-5 cells) and its regulation by thyroid stimulating hormone, which stimulated the secretory release of the proenzyme as well as of mature cathepsin B. Within one to two hours of stimulation with thyroid stimulating hormone, the cathepsin B activity associated with the plasma membrane increased significantly. This increase correlated closely with the localization of lysosomes in close proximity to the plasma membrane of cultured thyrocytes as well as with the thyroxine liberating activity of thyrocyte secretion media. These observations indicate that thyroid stimulating hormone induces the secretion of cathepsin B, which contributes to the extracellular release of thyroxine by thyrocytes.     

Cysteine proteinases mediate extracellular prohormone processing in the thyroid

Thyroglobulin, the precursor of thyroid hormones, is extracellularly stored in a highly condensed and covalently cross-linked form. Solublization of thyroglobulin is facilitated by cysteine proteinases like cathepsins B and K which are proteolytically active at the surface of thyroid epithelial cells. The cysteine proteinases mediate the processing of thyroglobulin by limited extracellular proteolysis at the apical plasma membrane, thereby rapidly liberating thyroxine. The trafficking of cysteine proteinases in thyroid epithelial cells includes their targeting to lysosomes where they become maturated before being transported to the apical plasma membrane and, thus, into the extracellular follicle lumen. We propose that thyroid stimulating hormone regulates extracellular proteolysis of thyroglobulin in that it enhances the rate of exocytosis of lysosomal proteins at the apical plasma membrane. Later, thyroid stimulating hormone upregulates thyroglobulin synthesis and its secretion into the follicle lumen for subsequent compaction by covalent cross-linking. Hence, cycles of thyroglobulin proteolysis and thyroglobulin deposition might result in the regulation of the size of the luminal content of thyroid follicles. We conclude that the biological significance of extracellularly acting cysteine proteinases of the thyroid is the rapid utilization of thyroglobulin for the maintenance of constant thyroid hormone levels in vertebrate organisms.     

Membrane-initiated actions of thyroid hormones on the male reproductive system

The presence of specific nuclear receptors to thyroid hormones, described in prepubertal Sertoli cells, implies the existence of an early and critical influence of these hormones on testis development. Although the mechanism of action thyroid hormones has been classically established as a genomic action regulating testis development, our research group has demonstrated that these hormones exert several effects in Sertoli cells lacking nuclear receptor activation. These findings led to the identification of non-classical thyroid hormone binding elements in the plasma membrane of testicular cells. Through binding to these sites, thyroid hormones could exert nongenomic effects, including those on ion fluxes at the plasma membrane, on signal transduction via kinase pathways, on amino acid accumulation, on modulation of extracellular nucleotide levels and on vimentin cytoskeleton. The evidence of the participation of different K(+), Ca(2+) and Cl(-) channels in the mechanism of action of thyroid hormones, characterizes the plasma membrane as an important microenvironment able to coordinate strategic signal transduction pathways in rat testis. The physiological responses of the Sertoli cells to hormones are dependent on continuous cross-talking of different signal transduction pathways. Apparently, the choice of the signaling pathways to be activated after the interaction of the hormone with cell surface binding sites is directly related to the physiological action to be accomplished. Yet, the enormous complexity of the nongenomic actions of thyroid hormones implies that different specific binding sites located on the plasma membrane or in the cytosol are believed to initiate specific cell responses.     

Response of the interrenal to adrenocorticotropic hormone after short-term thyroxine treatment of coho salmon (Oncorhynchus kisutch)

The effects of experimentally elevated plasma thyroxine levels on the subsequent response of interrenals of coho salmon (Oncorhynchus kisutch) to adrenocorticotropic hormone (ACTH) in vitro were examined. Animals were treated with thyroxine by immersion (200 micrograms/L) for 3 days, which resulted in physiological elevations in circulating thyroxine. In animals treated before the parr-smolt transformation was completed (early smolts), thyroxine had no effect on plasma cortisol levels but significantly enhanced the sensitivity of the interrenal to ACTH in vitro. In animals treated after the period of smoltification (postsmolts), plasma cortisol levels were significantly higher than those of controls; both experimental and control animals had plasma cortisol levels higher than normally observed at this stage of development. The response of the interrenals of thyroxine-treated postsmolts to ACTH in vitro was significantly lower than that of controls. Results from the experiments using early smolts are in agreement with studies in other vertebrates showing that thyroid hormones are involved in maintaining the normal functioning of corticosteroidogenic tissue and suggest that thyroid hormones may be involved in the activation of the interrenal that occurs during smoltification. The results obtained using postsmolts are more difficult to interpret because of the possibility that these animals were physiologically stressed by the treatment. Increased ACTH release in vivo resulting from stress may have led to a depression of interrenal sensitivity to ACTH in vitro and may have masked a refractoriness of the pituitary-interrenal axis to thyroxine.     

Timing of a single daily meal and diel variations of serum thyroxine,triiodothyronine and cortisol in goldfish carassius auratus

Two groups of goldfish were maintained on a 12L:12D photoperiod at 19°C and fed either at 0800 (light onset) or 1600 h daily. After 3 weeks, blood samples were taken at one of six times of day and the serum was analyzed for thyroxine, triiodothyronine, and cortisol by radioimmunoassay. Fish fed in the morning had significant diel variations in circulating titers of thyroxine and cortisol, those fed in the afternoon had significant variations of thyroxine and triiodothyronine. Meal feeding appeared to entrain, and phase shift, the cortisol rhythm but not the rhythms of thyroid hormones. These results are briefly discussed in light of the effects of timing the daily meal on weight gain in fish.     

Function of thyroid hormone transporters in the central nervous system

Background

Iodothyronines are charged amino acid derivatives that cannot passively cross a phospholipid bilayer. Transport of thyroid hormones across plasma membranes is mediated by integral membrane proteins belonging to several gene families. These transporters therefore allow or limit access of thyroid hormones into brain. Since thyroid hormones are essential for brain development and cell differentiation, it is expected that genetic deficiency of such transporters would result in neurodevelopmental derangements.

Scope of review

We introduce concepts of thyroid hormone transport into the brain and into brain cells. Important thyroid hormone transmembrane transporters are presented along with their expression patterns in different brain cell types. A focus is placed on monocarboxylate transporter 8 (MCT8) which has been identified as an essential thyroid hormone transporter in humans. Mutations in MCT8 underlie one of the first described X-linked mental retardation syndromes, the Allan–Herndon–Dudley syndrome.

Major conclusions

Thyroid hormone transporter molecules are expressed in a developmental and cell type-specific pattern. Any thyroid hormone molecule has to cross consecutively the luminal and abluminal membranes of the capillary endothelium, enter astrocytic foot processes, and leave the astrocyte through the plasma membrane to finally cross another plasma membrane on its way towards its target nucleus.

General significance

We can expect more transporters being involved in or contributing to in neurodevelopmental or neuropsychiatric disease. Due to their expression in cellular components regulating the hypothalamus–pituitary–thyroid axis, mutations and polymorphisms are expected to impact on negative feedback regulation and hormonal setpoints. This article is part of a Special Issue entitled Thyroid hormone signalling.     

Experimental studies on the annual cycles of thyroid and adrenocortical functions in relation to the reproductive cycle of drakes.

The annual variations in the basal plasma contents of testosterone, thyroxine and corticosterone have been measured in Peking drakes living outdoors, in Southern France. 10 The plasma testosterone titer underwent a more than 20-fold increase during the vernal reproductive period (March-April). In early June the circulating testosterone fell to near autumnal values, and the testosterone MCR was augmented. These were the first manifestations of the cessation of the vernal reproductive period. 20 The plasma thyroxine levels were minimal in autumn, moderately augmented (40%) in winter (January-March), but exhibited a 3-fold increase in early June. The resulting steep (13-fold) increase of the plasma thyroxine/testosterone ratio preceded the onset of the post-nuptial moult. 30 Modifications of testosterone secretion and clearance rate similar to those occurring in June were initiated in spring by i.m. injections of thyroxine at a dosage (1 mg/d) that induced June-July thyroxine plasma levels. On the other hand, an experimentally induced steep decrease of testosterone (castration) induced enhanced plasma thyroxine concentrations similar to the June values, while an induced (10 mg/d testosterone i.m.) hypertestosteronemia corresponding to the reproductive period depressed the plasma thyroxine levels. Strong reciprocal negative interactions between testis and thyroid might therefore afford a partial explanation of the peculiar thyroxine/testosterone imbalance that occurred in June immediately prior to the moult. 40 Cold-exposed "short-day" (December) ducks exhibited a marked increase in plasma thyroxine levels, while exposure of December ducks to "long days" (18L-6D) at 25 degrees C depressed the thyroxine titers. The inhibitory effect of "long days" on the blood level of thyroxine was further evident in castrated ducks. Exposure of "short day" ducks (December) to a combined treatment by "long days" (18L-6D) and cold (4 degrees C) produced an endocrine picture similar to the January-March pattern, i.e. highly increased testosterone plasma levels, but unaffected testosterone MCR, together with a moderate increase in plasma thyroxine concentration. 50 Corticosterone plasma concentrations increased during the reproductive season, as a result of a seasonally augmented binding-capacity of the CBG. Exogenous testosterone (10 mg/d) which induced spring-like circulating levels of male hormones, caused a similar increase in CBG-bound and total corticosterone levels. 60 In June the MCR of both corticosterone and aldosterone were elevated (as was the testosterone MCR). Similarly enhanced MCR of both corticosteroids were brought on in spring by an exogenous thyroxine treatment, leading to a June-like state of hyperthyroxinemia.     

Effect of alloxan and insulin immunoneutralization on circulating thyroid hormone levels in larval landlocked sea lampreys,petromyzon marinus

The effects of alloxan, an insulin (INS)-secreting cell toxin, and INS immunoneutralization on circulating levels of thyroid hormones (thyroxine, T(4); triiodothyronine, T(3)) were examined in larval landlocked sea lampreys, Petromyzon marinus. Animals were injected intraperitoneally with either (Experiment 1) saline (0.6%) or alloxan (20 or 200 microg/g body weight), or with (Experiment 2) normal rabbit serum or anti-lamprey INS. Alloxan (200 microg/g) decreased plasma T(3), but not T(4), in larvae by about 45-80%. Three, six, or nine hr after acute immunoneutralization of lamprey INS with anti-lamprey INS, plasma T(3) levels decreased by 13-30%, relative to controls. These data indicate that INS deficiency can regulate the thyroid system of larval lampreys. There is some suggestion that INS may mediate the metamorphic processes by modulating thyroid hormone concentrations.     

Chronic exposure to short photoperiod inhibits free thyroxine index and plasma levels of TSH, T4, triiodothyronine (T3) and cholesterol in female Syrian hamsters

1. Chronic exposure of female Syrian hamsters (Mesocricetus auratus) for 9 weeks to a short photoperiod (10L:14D) depressed the pituitary-thyroid axis as indicated by a drop in circulating titers of thyroid stimulating hormone (TSH), thyroxine (T4), triiodothyronine (T3) and the free thyroxine index (FT4I) compared to animals maintained under long photoperiodic conditions (14L:10D). 2. Short day treatment also reduced plasma cholesterol levels. 3. Neither plasma triglycerides, glucose nor growth hormone (GH) levels differed between hamsters exposed to short or long daily photoperiods.     

Rapid and transient reduction in circulating thyroid hormones following systemic antigen priming: implications for functional collaboration between dendritic cells and thyroid.

The thyroid hormones T(3) (tri-iodothyronine) and T(4) (thyroxine) are disseminated throughout the body via the circulation and are maintained across a range of physiological concentrations under the control of thyroid-stimulating hormone (TSH). T(3) (and T(4) after conversion to T(3)) influences many biological activities, including gene expression and protein synthesis, though little is known about the nature of pituitary-thyroid immune interactions. In the present study we show that serum T(3) and T(4) levels are sharply but transiently reduced during the first 24 h of systemic antigen exposure and that this is followed by suppressed levels of free T(4), after which there is rapid recovery to normal levels. Splenic dendritic cells, depending upon the stage of maturation/activation, were found to be a rich source of TSH, and CD11c(+) cells with dendritic cell morphology were present in the thyroid 1-3 days after antigen exposure. Moreover, antigen priming of hypophysectomized mice that are unable to make pituitary-derived TSH resulted in significant increases in circulating T(4), implying that compensation in the drop in thyroid hormones can be regulated from extrapituitary sources. These findings thus identify a novel set of immune-endocrine interactions that transpire during the early phase of antigen exposure, and they suggest that under appropriate conditions the immune system directly participates in the process of maintaining physiological homeostasis by contributing to the regulatory control of thyroid hormone activity.     

Thyroid hormones and their effects: a new perspective

The thyroid hormones are very hydrophobic and those that exhibit biological activity are 3',5',3,5-L-tetraiodothyronine (T4), 3',5,3-L-triiodothyronine (T3), 3',5',3-L-triiodothyronine (rT3) and 3,5',-L-diiothyronine (3,5-T2). At physiological pH, dissociation of the phenolic -OH group of these iodothyronines is an important determinant of their physical chemistry that impacts on their biological effects. When non-ionized these iodothyronines are strongly amphipathic. It is proposed that iodothyronines are normal constituents of biological membranes in vertebrates. In plasma of adult vertebrates, unbound T4 and T3 are regulated in the picomolar range whilst protein-bound T4 and T3 are maintained in the nanomolar range. The function of thyroid-hormone-binding plasma proteins is to ensure an even distrubtion throughout the body. Various iodothyronines are produced by three types of membrane-bound cellular deiodinase enzyme systems in vertebrates. The distribution of deiodinases varies between tissues and each has a distinct developmental profile. Thyroid hormones. (1) the nuclear receptor mode is especially important in the thyroid hormone axis that controls plasma and cellular levels of these hormones. (2) These hormones are strongly associated with membranes in tissues and normally rigidify these membranes. (3) They also affect the acyl composition of membrane bilayers and it is suggested that this is due to the cells responding to thyroid-hormone-induced membrane rigidificataion. Both their immediate effects on the physical state of membranes and the consequent changes in membrane composition result in several other thyroid hormone effects. Effects on metabolism may be due primarily to membrane acyl changes. There are other actions of thyroid hormones involving membrane receptors and influences on cellular interactions with the extracellulara matrix. The effects of thyroid hormones are reviewed and appear to b combinations of these various modes of action. During development, vertebrates show a surge in T4 and other thyroid hormones, as well as distinctive profiles in the appearance of the deiodinase enzymes and nuclear receptors. Evidence from the use of analogues supports multiple modes of action. Re-examination of data from th early 1960s supports a membrane action. Findings from receptor 'knockout' mice supports an important role for receptors in the development of the thyroid axis. These iodothyronines may be better thought of as 'vitamone'-like molecules than traditional hormonal messengers.     

Membrane reception of thyroid hormones]

Comparative and competitive analyses of thyroxine (T4) and triiodothyronine (T3) binding to highly purified rat liver, brain and lung cell plasma membranes were carried out. The dependence of hormone binding on the time, temperature and concentration was studied. The effects of trypsin and partial delipidation on the binding parameters of thyroid hormones were investigated. Two thyroid hormone-binding sites were detected in cell plasma membranes of all tissues under study. The maximal binding of T4 to rat liver membranes and the maximal binding of T3 to rat brain membranes was observed in all experiments, the affinity for T3 being higher than that for T4. An important role of both protein and lipid components of plasma membranes in the membrane reception of thyroid hormones is proposed.     

Thyroid Hormone Enhances the Formation of Synapses Between Cultured Neurons of Rat Cerebral Cortex

1. Thyroid hormones play important roles in the development of the brain. Increasing evidence suggests that the deprivation of thyroid hormones in the early developmental stage causes structural and functional deficits in the CNS, but the precise mechanism underlying this remains elusive. In this study, we investigated the effects of thyroid hormones on synapse formation between cultured rat cortical neurons, using a system to estimate functional synapse formation in vitro. 2. Exposure to 10(-9) M thyroid hormones, 3,5,3'-triiodothyronine or thyroxine, caused an increase in the frequency of spontaneous synchronous oscillatory changes in intracellular calcium concentration, which correlated with the number of synapses formed. 3. The detection of synaptic vesicle-associated protein synapsin I by immunocytochemical and immunoblot analysis also confirmed that exposure to thyroxine facilitated synapse formation. 4. The presence of amiodarone, an inhibitor of 5'-deiodinase, or amitrole, a herbicide, inhibited the synapse formation in the presence of thyroxine. 5. In conclusion, we established a useful in vitro assay system for screening of miscellaneous chemicals that might interfere with synapse formation in the developing CNS by disrupting the thyroid system.     

Antibodies to nuclear thyroid hormone-binding proteins. Antibody characterization and immunofluorescent localization

To further investigate the mechanism by which thyroid hormones regulate target cell function, we have prepared and partially characterized antibodies to highly purified nuclear thyroid hormone-binding proteins (NTBP). NTBPs were prepared from bovine liver nuclear extracts by bio-specific elution from an affinity gel containing immobilized 3,5,3'-triiodo-L-thyronine (T3). Antibodies (Ab) raised to NTBP in BALB/c mice were assayed for Ab-NTBP complex formation on HPLC TSK SW3000 molecular exclusion gels and found to be species-specific and non-cross-reactive with serum thyroid hormone-binding proteins. Most of the antibody activity was directed against two fractions of molecular weight (MW) 89 000 and 53 000, which were associated with thyroxine (T4)-binding activity. The 89 000 D T4-binding activity was shifted to a higher MW complex when incubated with specific antibody. Indirect immunofluorescence showed antibody activity against discrete, clumped chromatin sites, nuclear envelope and plasma membrane in hepatocytes. Intense fluorescence was also observed in the cells lining the hepatic sinusoids and in the cytoskeleton of bovine aortic endothelial cells in culture. The data suggest that thyroid hormone target cells contain extranuclear loci that share antigenic sites with NTBP and may also represent specific NTBP-like sites of thyroid hormone binding.     

Time course of changes in plasma concentrations of the growth related hormones during protein restriction in the domestic fowl (Gallus domesticus)

Experiments were conducted to evaluate the possible role of circulating growth hormones triiodothyronine (T3), thyroxine (T4), and insulin-like growth factor I (somatomedin-C; IGF-I) in the elevation of plasma growth hormone (GH) which occurs in protein-restricted chickens. Plasma hormone changes were determined over a 2-week period of protein depletion by feeding a 5% protein diet as well as a similar period of protein repletion with a 20% protein diet. The rise in plasma GH was observed in two separate studies. Plasma concentrations of T4, T3, and IGF-I were all depressed in protein-restricted chicks prior to or concurrent with the GH elevation. In the protein repletion time course study, T4 and T3 concentrations were normalized prior to or concurrent with plasma GH normalization. However, IGF-I concentrations in repleted chicks did not return to control levels until after normal levels of GH were observed. These data suggest that thyroid hormones may play a greater role in the regulation of GH secretion during periods of malnourishment than IGF-I; the latter being currently thought to be a peripherally circulating inhibitor of GH release in animals.     

Molecular Mechanisms of the Relationship between Thyroid Dysfunctions and Diabetes Mellitus

Type 1 and type 2 diabetes mellitus (DM) are known to increase the incidence of thyroid gland (TG) dysfunctions. The review addresses the literature data and our experimental results on the molecular mechanisms that underlie thyroid disorders under DM. Most important of these mechanisms are the attenuation of thyrocyte adenylyl cyclase signaling system sensitivity to thyroid-stimulating hormone, the decrease in the number of thyroid hormone receptors in peripheral tissues, and the decline in activity as well as changes in the ratio of different deiodinase forms in these tissues. Decreased activity of D2 deiodinases, which convert thyroxine into the active form of triiodothyronine, is associated with the development of insulin resistance, while decreased activity of D3 deiodinases, which catalyze inactivation of triiodothyronine in pancreatic β cells, suppresses insulin secretion and leads to insulin deficiency. Thus, both the excess and the deficiency of thyroid hormones can entail diabetic pathology. Identification of thyroid disorders is of utmost importance for elaborating novel approaches to treat and prevent thyroid diseases associated with type 1 and type 2 DM.     

Expression of type II iodothyronine deiodinase marks the time that a tissue responds to thyroid hormone-induced metamorphosis in Xenopus laevis

The thyroid gland synthesizes thyroxine (T4), which passes through the larval tadpole's circulatory system. The enzyme type II iodothyronine deiodinase (D2) converts thyroxine (T4) to the active hormone 3,5,3'-triiodothyronine (T3) in peripheral tissues. An early response to thyroid hormone (TH) in the Xenopus laevis tadpole is the stimulation of cell division in cells that line the brain ventricles, the lumen of the spinal cord, and the limb buds. These cells express constitutively high levels of D2 mRNA. Exogenous T4 induces early DNA synthesis in brain, spinal cord, and limb buds as efficiently as T3. The deiodinase inhibitor iopanoic acid blocks T4- but not T3-induced cell division. At metamorphic climax, both TH-induced cell division and D2 expression decrease in the brain. Then D2 expression appears in late-responding tissues including the anterior pituitary, the intestine, and the tail where cell division is reduced or absent. Therefore, constitutive expression of D2 occurs in the earliest target tissues of TH that will grow and differentiate, while TH-induced expression of D2 takes place in late-responding tissues that will remodel or die. This pattern of constitutive and induced D2 expression contributes to the timing of metamorphic changes in these tissues.     

Structure of the thyroid gland, serum thyroid hormones, and the reproductive cycle of the Atlantic stingray, Dasyatis sabina.

This study examines the role of the thyroid gland in the control of reproduction in the viviparous Atlantic stingray, Dasyatis sabina. Thyroid activity in individuals in different reproductive stages was assessed both by microscopic examination of the gland, and by analysis of circulating levels of thyroid hormones from the same individuals. The thyroid gland is a cylindric organ, embedded in a connective tissue capsule, and composed of follicles, i.e., monolayer spheres of thyroid epithelial cells. Stingray follicular cells possess several characteristic features, namely apical cilia and a well-developed endoplasmic reticulum. Cells vary in size and shape, according to the activity of the gland. No structural differences were observed between the thyroid glands of the two sexes. Both thyroid hormones, triiodothyronine, [T(3)], and thyroxine, [T(4)], were detected in the serum of all animals examined. Levels ranged from 1.3-2.6 microg/100 ml for total T(4), and from 1.2-2.6 ng/ml for total T(3). The T(4) levels did not vary significantly in any group. Immature individuals and females undergoing oogenesis had the lowest levels of circulating T(3) and mature females from ovulation throughout gestation had high thyroid gland activity and high levels of circulating T(3). J. Exp. Zool. 284:505-516, 1999.     

Nongenomic effects of thyroid hormones on the immune system cells: New targets, old players

It is now widely accepted that thyroid hormones, l-thyroxine (T(4)) and 3,3',5-triiodo-l-thyronine (T(3)), act as modulators of the immune response. Immune functions such as chemotaxis, phagocytosis, generation of reactive oxygen species, and cytokine synthesis and release, are altered in hypo- and hyper-thyroid conditions, even though for many immune cells no clear correlation has been found between altered levels of T(3) or T(4) and effects on the immune responses. Integrins are extracellular matrix proteins that are important modulators of many cellular responses, and the integrin αvβ3 has been identified as a cell surface receptor for thyroid hormones. Rapid signaling via this plasma membrane binding site appears to be responsible for many nongenomic effects of thyroid hormones, independent of the classic nuclear receptors. Through the integrin αvβ3 receptor the hormone can activate both the ERK1/2 and phosphatidylinositol 3-kinase pathways, with downstream effects including intracellular protein trafficking, angiogenesis and tumor cell proliferation. It has recently become clear that an important downstream target of the thyroid hormone nongenomic pathway may be the mammalian target of rapamycin, mTOR. New results demonstrate the capability of T(3) or T(4) to induce in the short time range important responses related to the immune function, such as reactive oxygen species production and cell migration in THP-1 monocytes. Thus thyroid hormones seem to be able to modulate the immune system by a combination of rapid nongenomic responses interacting with the classical nuclear response.     

Harderian glands of golden hamsters: morphological and biochemical responses to thyroid hormones

Summary Manipulation of circulating levels of thyroid hormones modifies Harderian gland structure and porphyrin concentrations in male and female golden hamsters. Specifically, thyroxine (T₄) and triiodothyronine (T₃) induce the morphological conversion of the Harderian glands of females to approximate those of the male. Further, porphyrin concentrations are markedly decreased by this treatment. This effect occurs in ovariectomized animals as well, indicating that the gonads are not involved. Suppression of thyroid function by potassium perchlorate (KClO₄) drastically reduces Harderian gland weight in both males and females. However, KClO₄ decreases porphyrin levels in the Harderian glands of females and increases it in the male. Concurrently, KClO₄ also induces a morphological conversion of the Harderian glands of males to the female type. This effect is evident in photoperiods of either 14:10 (h) or 8:16 (h).