1 CONTACT Julie Cecere juliet@vt.edu © 2023 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (http:// creativecommons.org/licenses/by-nc/4.0/), permitting all non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Citation line: Clinical Theriogenology 2023, 15, 9916, http://dx.doi.org/10.58292/CT.v15.9916 Review Report Hypothyroidism in canine reproduction† Julie Cecere Department of Small Animal Clinical Sciences, Virginia-Maryland College of Veterinary Medicine, Blacksburg, VA, USA Abstract Hypothyroidism is one of the most commonly diagnosed endocrine diseases in dogs. It has been implicated for many reproductive difficulties in breeding dogs; however, only a few studies have investigated the involvement of thyroid hormones in reproduction. Due to intricacies related to appropriate testing, breeding animals may be placed on thyroid supplementation without justification. The disease, testing, heritability, and animals that may need supplementation are discussed. Additionally, the assumption/appro- priateness of breeding animals that are truly hypothyroid is questioned. Keywords: Hypothyroid, reproduction, endocrine, infertility Pathophysiology and diagnostic challenges Hypothyroidism is the most common endocrine disease diag- nosed in canine practice, although true disease prevalence is unknown.1,2 Over 90% of cases are due to an immune-medi- ated pathogenesis characterized by lymphocytic thyroiditis or idiopathic follicular atrophy.1,3 Rarely hypothyroidism is due to a secondary or tertiary pathogenesis, but those account for  < 5% of cases.3 Gonadectomized animals appear to be over-represented and in euthyroid animals circulating thyroid hormone concentrations decrease as dogs age. Disease onset is insidious and can take months to years before any clinical signs develop.3 Average age at onset of clinical signs is 7 years and clinical signs overlap with other endocrinopathies in many cases.1,3 Obesity is present in ~ 50% of cases, followed by der- matologic aberrations (present in 60 - 80% of dogs).3 Dermatologic conditions commonly include alopecia, seborrhea, poor hair coat, and lethargy or a dull menta- tion.1,3 Several breeds are at an increased risk of developing hypothyroidism, mainly, Golden Retrievers and Doberman Pinschers. Beagles and Borzois have heritable lymphocytic thyroiditis.4,5 Breeds that have a high incidence of thyroglobulin autoantibodies (TgAA) (antithyroglobulin antibodies) include Boxers, Dalmatians, Giant Schnauzers, Great Danes, Setters, and Old English Sheepdogs, among others.1,3–7 Diagnosis should be based on serologic testing and not on clinical signs alone as there are many diseases and medica- tions, or even breed differences, that contribute to altered cir- culating thyroid hormone concentrations. Accurate testing should be utilized before supplementation is considered. Both 3,5,3’-triiodothyronine (T3) and thyroxine (T4) are highly protein bound and subject to drastic changes in con- centrations of albumin and thyroxin-binding globulin.1 Many diseases can also falsely lower circulating T4 concentrations and yield a diagnosis of euthyroid-sick syndrome. Specific drugs can alter circulating serum concentrations of thyroid hormones; therefore, any animals being treated with sulfon- amides, glucocorticoids, and/or phenobarbital should be tested after a ‘washout’ period, or if this is not practical, the results should be interpreted carefully.1,3 There are lower circulating T4 concentrations in specific breeds, including Greyhounds, Whippets, Scottish Deerhounds, Basenji, Alaskan sled dogs, and Sloughis; these breeds have breed-spe- cific reference ranges1,3,8–11 Importantly, euthyroid dogs have lower T4 concentrations at some time point during the day, a major consideration when testing is recommended.1 Thyroxine concentrations are lower in > 90% of hypothyroid dogs. Most general practitioners can use ‘in-house’ serum T4 concentration tests. This test is sensitive, but is not specific for canine hypothyroidism and should be avoided as a single test.1,3 The most accurate single test to diagnose hypothyroid- ism is free thyroxine (fT4) by equilibrium dialysis.3 Thyroid †Presented at the 2023 Society for Theriogenology conference, published after peer review. mailto:juliet@vt.edu http://creativecommons.org/licenses/by-nc/4.0/ http://creativecommons.org/licenses/by-nc/4.0/ http://dx.doi.org/10.58292/CT.v15.9916 2 Citation line: Clinical Theriogenology 2023, 15, 9916, http://dx.doi.org/10.58292/CT.v15.9916 autoantibodies, and to a lesser extent nonthyroidal illness, do not affect fT4 concentrations. Combining serum concentra- tions of thyroid hormones increases success in identifying ani- mals with true hypothyroidism. Using thyrotropin (TSH) information with T4 or fT4 typically yields a 98% specificity.1,3 In sighthounds, T3 concentrations may be useful because their T4 concentrations are typically outside reference ranges.3 Thyroglobulin autoantibodies (TgAA) form as the dog’s immune system reacts to their thyroglobulin and are present in approximately 50% of all hypothyroid dogs.3 Animals that are positive for TgAA but with normal thyroid function tests should be retested annually, as they have a higher risk of actu- ally developing clinical disease than do euthyroid dogs with negative TgAA.12 Reproductive manifestations Hypothyroidism in humans has been studied extensively and is currently ongoing. There are known reproductive manifes- tations that affect both women and men. Reports include abnormal cyclicity or anovulation, infertility, loss of preg- nancy in women, and loss of libido or fertility in men.13,14 Women who did not receive adequate supplementation of thyroid hormones or who have had impaired iodine intake during pregnancy had children with substantial effects,15,16 particularly impaired cognitive function. The relation between thyroid hormone supplementation and diet is being elucidated in women. The effect of thyroid hormones on both in vitro and in vivo pregnancy rates are also being stud- ied. Men who were > 35 years and subclinically hypothyroid had decreased clinical pregnancy rate when compared with their euthyroid counterparts.14 Although these findings are valid, it is important to notice that human and canine preg- nancies are very different. Humans have higher concentra- tions of thyroxine binding globulins (TBG) and placental deiodinases; both drastically increase the metabolic demand for thyroid hormones during pregnancy.17 Humans also pro- duce human chorionic gonadotropin (hCG) in pregnancy, which has substantial thyrotropic activity.17 Broad assump- tions based on human literature surrounding hypothyroid disease and its reproductive effects in dogs should be avoided due to these differences. Fewer studies were performed on dogs, but despite the lack of data, a plethora of reproductive diseases are ‘blamed’ on hypo- thyroidism. Reports have implicated hypothyroidism for cycle aberrations, abortion, infertility, and stillbirth.3,18–20 To date only a handful of prospective studies have been performed specifically highlighting the reproductive manifestations of hypothyroidism in female dogs. A review of 204 dog breeds suspected to have heritable hypothyroidism failed to demon- strate any significant difference in circulating thyroid hormone concentrations among normal, infertile, and subfertile male and female individuals.7 Additionally, a large prospective study that utilized fertile, previously proven mongrel female dogs, in which hypothyroidism was induced with 131I treatment, shed some light on pregnancy and hypothyroidism. During the short-term portion of the study, all dogs cycled normally, became pregnant and carried their litters to term.21 Furthermore, there was no difference in litter size, although pups born to hypothyroid dogs weighed less and experienced more peripar- turient mortality. Weight gain of pups was equal between the groups to day 28 postpartum. The increased periparturient mortality was attributed to possible prolonged uterine contrac- tions that could be due to generalized muscle weakness in hypothyroid patients.21 A second article, from the same cohort, evaluated the effects of prolonged experimentally induced hypothyroidism on repro- duction. All experimental animals were profoundly hypothy- roid and had typical clinical manifestations of weight gain, hair coat changes, weakness, and lethargy.22 Control and hypothy- roid female dogs were bred twice during this portion of the study, once without supplementation and once with levothy- roxine supplementation. Many variables were monitored to document the effects of long-term uncontrolled hypothyroid- ism during pregnancy (2nd breeding) and the effects of supple- mentation and pregnancy after clinical signs were controlled (3rd breeding). The interestrus interval (IEI), pregnancy rates, and duration of pregnancy were not different between groups.22 Following the second breeding, only half of the hypothyroid dogs became pregnant, highlighting profound long-term effects of hypothyroidism.22 Ultimately, levothyroxine supplementa- tion eliminated any differences between the groups at the 3rd breeding.22 A final study evaluated circulating thyroid hor- mones present in the hypothyroid female dogs from the 3rd breeding of the same cohort. No gestational adjustments in thy- roid supplementation were required for inducing a euthyroid state due to no difference in thyroid hormone concentrations or progesterone concentrations between groups, which is dras- tically different from hypothyroid women.17 Another study evaluated dogs that had aborted their litters after 4th week of pregnancy. This study measured only T4 and progesterone and failed to conduct further diagnostics to work up the cause of abortion. This leads to only a weak argument for a direct effect of T4 being a singular cause of abortion.23 Conflicting reports exist regarding hypothyroidism and its effects on male dog fertility. Specific colonies containing sim- ilar genetic makeup had both hypothyroidism and infertil- ity2,5,24 Due to the specifics of genetic makeup of these colonies, one might argue that the infertility observed may be due to limited genetic diversity rather than a direct effect of low circu- lating thyroid hormones. One 1997 study reported that induced hypothyroidism did affect male dog fertility but a later (2009) in vivo study reported no statistical difference in thyroid function between fertile and infertile dogs.7,25 Another 1999 study utilized induced hypothyroidic dogs and noted no change in libido, daily sperm output, motility parameters, nor sperm morphology between control and hypothyroid dogs.26 A few major points from these studies are important. No experimental study to date has reproduced reproductive fail- ure following an immunogenic hypothyroidism mechanism, the naturally occurring disease in canine populations. Manifestations of any reproductive failure have required elim- ination of all functional thyroid tissue within the animal. This raises the question of whether the reproductive effects observed are due to the lack of circulating hormones or a chronic inflam- matory state produced by profound endocrine disease and associated endocrine disruption. Furthermore, many of the animals in the studies were obese, which is an independent cause of reproductive failure in many species. Despite these drawbacks, it is fair to conclude that primary hypothyroidism and its effects on reproduction in canine patients are only weakly associated. Hypothyroidism should be approached cautiously as a cause of clinical infertility or subfertility. Heritability and testing breeding animals Many breeds are labeled as ‘at risk’ for developing hypothyroid- ism. Consequently, there is a high suspicion that a  heritable http://dx.doi.org/10.58292/CT.v15.9916 Citation line: Clinical Theriogenology 2023, 15, 9916, http://dx.doi.org/10.58292/CT.v15.9916 3 and genetic component may be a factor in developing the dis- ease. The insidious nature of the disease and difficulties sur- rounding its definitive diagnosis have left heritability studies as inconclusive. This leads to the conclusion that the underlying genetics are likely a polygenic trait that varies from one breed to another and that developing hypothyroidism is also heavily influenced by environmental factors.6 Penetrance of the offend- ing alleles is also likely to have a role in the expression of clini- cal disease. Despite these drawbacks, there are testing mechanisms that help screen potential breeding stock and assist breeders and veterinarians in choosing animals less likely to pass hypothyroidism on to their offspring. The use of spe- cific, accurate testing mechanisms is very important. The use of a single T4 or even T4 combined with TSH should be avoided in screening broodstock. A full panel including fT4 by equilibrium dialysis, T4, cTSH, TgAA, and T3/fT3 should be used. Animals that are exhibiting signs of concurrent disease should not be tested until the disease state has been resolved. Testing should begin around 1 year of age and should continue every 1–2 years while the animal is breeding or until the animal reaches 8–10 years of age. This allows for identification of animals that may develop subclinical disease over the course of their life, despite never having fulminant clinical signs. It is important to remem- ber that hypothyroidism is a disease that is easily and success- fully treated. Animals that may be TgAA positive and remain in the normal range of other thyroid function tests should not be eliminated from the breeding pool, because eliminating them can lead to diminished genetic diversity for many other traits. Breeding animals must be taken as a whole, and breeders should avoid ‘throwing the baby out with the bathwater’, espe- cially for a disease that is so easily treated. Levothyroxine supplementation Only animals that had appropriate testing and elimination of all other potential causes of infertility should be considered for levothyroxine supplementation. Typically, these dogs have been bred several times, conceived, and failed to deliver or lost a large portion of their litter to resorption during the 4th to 6th week of pregnancy. Documentation of similar history with appropriate diagnostics, serial ultrasonographic examinations of the reproductive tract, infectious disease testing, complete blood count and biochemical profiles are warranted before considering supplementation. This includes a thorough history including pharmaceuticals used (preventatives and antibiotics), appropriate IEI, breeding method(s) and timing, fertility of the sire, genetic evaluation of the breeding pair, and any other comorbidities. Intrauterine cytology, culture and/or biopsy should be performed in early proestrus, positive results should be taken seriously (addressed and discussed), and automatic pharmaceutical treatment is avoided. If the animal is timed appropriately, bred to a proven, fertile sire and again fails to carry to term, then it is time to consider supplementation. Supplementing typically causes female dogs to be hyperthy- roid, and this should only be done if a clear justification exists. Supplementation needs to be fully discussed prior to imple- mentation. This discussion should include the genetic possi- bility of needing exogenous hormone therapies to maintain pregnancy in their lines. Terminal, performance breeding is well accepted, but a kennel that is experiencing reproductive failure should carefully consider eliminating this type of ani- mal as a brood female dog. Once daily treatment of oral levothyroxine (0.02 mg/kg) is sufficient for supplementation. Treatment for female dogs should start as soon as the veterinarian feels that they fit the criteria discussed above (during pregnancy) or at the begin- ning of her next estrous cycle and will be bred. All pregnancies in these dogs are considered high risk and should be monitored carefully and often. A tapered withdrawal over 2 weeks should be initiated immediately after whelping. Typically, these dogs do not have any other confounding factors, and are without evidence of endometritis, pyometra, or metritis, hypoluteoid- ism, or other causes of pregnancy failure. Breeders often demand supplementation well before a clear cause is identi- fied; however, there is sufficient evidence that the thyroid gland must be profoundly affected before severe reproductive problems manifest. This provides a clear path to appropriate and science-based supplementation. There may be another mechanism in which thyroid supplementation will assist these specific cases, but the exact role has not been elucidated and is the very reason for caution. This calls for the judicious use of thyroid supplementation despite what is known in breeder community as ‘breeder lore’. Conclusion Hypothyroidism is a common disease of our canine population and is difficult to definitively diagnose. Furthermore, evidence of direct cause and effect of hypo- thyroidism and reproductive failure in a natural disease state is lacking. Reproductive veterinarians need to weigh all infectious, genetic, and husbandry causes of reproduc- tive failure diligently and in a systematic approach before diagnosis and supplementation of thyroid hormones are implemented. Conflict of interest The author declares that she has no conflict of interest. References 1. Bruyette DS: Canine hypothyroidism. In: Bruyette DS, Bexfield N, Chretin JD, et al., editors. Clinical Small Animal Internal Medicine,  vol. 1, John Wiley & Sons, 2020, p. 71–74. doi: 10.1002/9781119501237 2. Fritz TE, Lombard LS, Tyler SA, et al: Pathology and familial incidence of orchitis and its relation to thyroiditis in a closed beagle colony. Exp Mol Pathol 1976;24:142–158. doi: 10.1016/0014-4800(76)90002-2 3. Panciera D: Hypothyroidism in dogs. In: J Rand, editor. Clinical Endocrinology of Companion Animals. 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