This work was conducted at Ghent University, Belgium
This article did not receive support from any grant.
This review has not been presented in any meeting.
Corresponding author: M. Campos, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium; e-mail: firstname.lastname@example.org.
Recombinant human thyrotropin (rhTSH) was developed after bovine thyrotropin (bTSH) was no longer commercially available. It was approved by the Food and Drug Administration (FDA) and the European Medicines Agency (EMEA) as an aid to diagnostic follow-up of differentiated thyroid carcinoma in humans and for thyroid remnant ablation with radioiodine. In addition, rhTSH is used in human medicine to evaluate thyroid reserve capacity and to enhance radioiodine uptake in patients with metastatic thyroid cancer and multinodular goiter. Likewise, rhTSH has been used in veterinary medicine over the last decade. The most important veterinary use of rhTSH is thyroidal functional reserve testing for the diagnosis of canine hypothyroidism. Recent pilot studies performed at Ghent University in Belgium have investigated the use of rhTSH to optimize radioiodine treatment of canine thyroid carcinoma and feline hyperthyroidism. Radioiodine treatment optimization may allow a decreased therapeutic dosage of radioiodine and thus may improve radioprotection. This review outlines the current uses of rhTSH in human and veterinary medicine, emphasizing research performed in dogs and cats, as well as potential future applications.
In human medicine, recombinant human thyrotropin1 (rhTSH) is used mainly in the diagnostic follow-up of differentiated thyroid carcinoma (DTC) and in postsurgical thyroid remnant ablation with radioiodine-131 (131I). It has now been over a decade since rhTSH became commercially available. Since then, various uses of rhTSH also have been investigated for veterinary medicine.
In veterinary medicine, rhTSH is mainly used in the diagnosis of canine hypothyroidism. Canine primary hypothyroidism may be caused by idiopathic follicular atrophy or autoimmune lymphocytic thyroiditis. The latter is comparable to atrophic thyroiditis in humans. In dogs and humans, thyroid carcinoma is mainly of follicular cell origin. Although the treatment of choice for canine thyroid carcinoma is surgical excision, unresectable tumors may be treated with external beam radiation or, as in humans, with 131I therapy. Hyperthyroidism is the most commonly diagnosed endocrine disorder in geriatric cats and resembles toxic nodular goiter in humans. The treatment of choice in both species is 131I therapy. The number of dogs and cats treated with 131I has increased dramatically over the past few years, with specialized treatment centers arising all over the world.
Given the similarities in thyroid pathology and its management among humans, dogs, and cats, it is interesting to compare the roles of rhTSH in the diagnosis and treatment of thyroid disease across these species. The extensive research on rhTSH in 131I therapy optimization of human DTC and nodular goiter provides an important perspective on the future of rhTSH in veterinary medicine. Humans, dogs, and cats share the same environment, and decreased exposure to radioactivity after 131I therapy benefits humans both as patients and as owners. This review outlines the development and use of rhTSH in human and veterinary medicine, emphasizing recent research in dogs and cats and possible future applications.
Thyrotropin, or thyroid-stimulating hormone (TSH), is a glycoprotein secreted by thyrotrophs in the adenohypophysis. It is composed of α and β subunits. The α subunit has 2 oligosaccharide chains and is common to other glycoprotein hormones; the β subunit has 1 oligosaccharide chain and is hormone-specific. Although the exact amino-acid sequence of the β subunit varies among species, there is biologic cross-reactivity, such that the TSH of 1 species will stimulate the thyroid follicular cells of another species.[1, 5] The sequences of the α and β subunits of feline TSH are 96 and 94% homologous to canine TSH and 68 and 88% homologous to human TSH.
Thyrotropin binds to a membrane TSH G protein-coupled receptor on the surface of follicular thyroid cells and constitutes the most important stimulus for proliferation, differentiation, and metabolic activity in these cells. This binding activates adenyl cyclase, causing an increase in the intracytoplasmic concentrations of adenosine 3′,5′-cyclic monophosphate (cAMP), after which phosphorylation of protein kinases takes place. This interaction triggers a cascade of reactions, leading to increased iodide uptake and synthesis and secretion of triiodothyronine (T3), thyroxine (T4), and thyroglobulin (Tg). Within 3–6 hours after TSH binds, endocytosis of colloid increases, and preformed thyroid hormone is released from the colloid into the bloodstream; hormone concentrations reach a maximum after 24 hours. When TSH stimulation persists, increased expression and functionality of the Na/I symporter (NIS) lead to increased uptake and organification of iodine, which peaks after 72 hours[8-10] (Fig 1).
Development of rhTSH
The standard treatment for human DTC is total or near-total thyroidectomy, followed by destruction of all remaining thyroid cells with 131I therapy (thyroid remnant ablation) and long-term levothyroxine supplementation. Levothyroxine supplementation is not only performed to treat iatrogenic hypothyroidism but also to suppress endogenous TSH, which constitutes a potential stimulus for tumor growth. Indeed, experimental evidence shows that TSH also may regulate the proliferation of neoplastic thyroid cells in dogs. TSH-suppressive therapy prolongs relapse-free survival in people with DTC, but temporarily high concentrations of TSH are required in the postsurgical follow-up of thyroid cancer.[12-15] Follow-up of DTC in human patients is based on cervical ultrasound examination, serum Tg concentration, and 131I whole-body scan (WBS). High plasma concentrations of TSH are necessary for appropriate iodine uptake by normal and neoplastic thyroid cells and, consequently, for a sensitive WBS. High concentrations of TSH also are necessary to increase the sensitivity of Tg determination as a marker for tumor persistence and recurrence. To achieve high concentrations of endogenous TSH (> 30 mIU/mL), levothyroxine supplementation must be discontinued for 4–6 weeks. However, thyroid hormone withdrawal (THW) typically results in severe hypothyroidism, which has a serious impact on the patient's quality of life. Alternatively, levothyroxine supplementation may be continued. In this case, exogenous TSH administration would be used before WBS and serum Tg determination. Initially, bovine TSH (bTSH) and extractive-human TSH (obtained from cadaver pituitary glands) were used for this purpose.
In the early 1990s, the use of bTSH in humans had been largely abandoned because of allergic reactions and decreased efficacy after repeated administrations caused by the development of anti-bTSH antibodies.[17-20] Extractive TSH from human cadavers was abandoned early because of its limited availability and potential for transmission of Jacob-Creutzfeldt disease.[21, 22] The need created by these difficulties led to the development of rhTSH.
The genes encoding the α and β subunits of TSH were indentified and cloned in the 1980s. This success was followed by the expression of an intact human TSH heterodimer in Chinese hamster ovary cells transfected with plasmids carrying hCGα and hTSHβ complementary DNA.[24, 25] Purification and large-scale production of rhTSH was achieved independently by 2 laboratories (Genzyme; USA and Shionogi Research Laboratories, Japan), which produced rhTSH with identical sequences despite having different sources of complementary DNA. The amino-acid sequence of rhTSH is identical to that of endogenous human TSH, although there is less glycosylation and more sialic acid.
Recently, concerns have been raised by the FDA regarding rhTSH vial contamination during manufacturing. These problems led to a temporary halt of production and a global shortage of rhTSH. It is still unclear when the normal supply of this drug will resume.
Use of rhTSH in Human Medicine
The efficacy of treating humans with DTC and the specificity of postsurgical Tg measurement can be increased by 131I ablation of thyroid remnants after thyroidectomy. With 131I ablation, all remaining thyroid tissue that may contain tumor cells is destroyed, and consequently measurement of Tg becomes a highly specific marker for recurrent or persistent disease. Preparation of patients for thyroid remnant ablation after thyroidectomy traditionally involved THW. However, as had been seen in diagnostic follow-up, THW results in clinical hypothyroidism and impaired quality of life.[29, 30] After rhTSH was first approved for the follow-up of DTC, several studies demonstrated it could also stimulate 131I uptake by the thyroid remnants after thyroidectomy, facilitating complete ablation.[31-33] Preparation of thyroid remnant ablation with rhTSH is associated with a higher quality of life, lower radiation exposure to the blood and lower radiotoxicity whereas ablation rate and tumor recurrence are comparable to THW.[34-36]
Recombinant human TSH also can be used as an adjuvant to 131I treatment of metastatic and persistent DTC. If patients with metastatic DTC cannot achieve adequate plasma TSH concentrations after THW or experience serious depression associated with iatrogenic hypothyroidism, rhTSH may be used compassionately to increase 131I uptake by tumor tissue.[37, 38]
Recombinant human TSH also is valuable in the treatment of multinodular goiter (MNG). Toxic MNG is characterized by multiple nodular enlargement of the thyroid gland and hyperthyroidism, resembling feline hyperthyroidism. 131I is the treatment of choice and recent studies have shown that pretreatment with rhTSH allows increased thyroidal uptake of 131I, reduction of the therapeutic dose of 131I, and decreased radiation delivery to extrathyroidal tissues.[39-45] A summary of the uses of rhTSH in human medicine is provided in Table 1.
Table 1. Uses of rhTSH in humans (adapted from Duntas et al)
The similarities between human and canine thyroid cancer, and between human toxic MNG and feline hyperthyroidism, provide opportunities for comparative research on these diseases. As in humans, rhTSH potentially may play an important role in optimizing 131I therapy for canine thyroid carcinoma and feline hyperthyroidism.
Use of rhTSH in Veterinary Medicine
An overview of rhTSH research in veterinary medicine is presented in Table 2.
Table 2. Overview of veterinary literature investigating the use of rhTSH
BW, body weight; TT4, total thyroxine; NTI, nonthyroidal illness; ~, estimation.
The diagnosis of canine hypothyroidism typically is based on compatible clinical signs, decreased total thyroxine (TT4) concentration, and increased cTSH concentration. However, approximately one-third of hypothyroid dogs do not have the typical increase in serum TSH concentration.[46, 47] In rare cases, this may be because of secondary or tertiary hypothyroidism, but it is now thought that in most cases, this phenomenon might result from a progressive TRH-receptor desensitization of pituitary thyrotrophs caused by persistent stimulation during primary hypothyroidism. Whatever the cause of the low sensitivity of TSH measurement, patients with low TT4 and normal TSH concentrations present a diagnostic challenge because dogs with nonthyroidal illness (NTI) and dogs receiving thyroid-suppressing drugs also may have low TT4, as well as occasionally low free T4 (fT4) and a normal TSH concentration. In these cases, additional tests such as TSH stimulation, thyroid ultrasound examination and thyroid scintigraphy may help discriminate NTI from hypothyroidism. TSH stimulation allows thyroid functional reserve testing without need for sedation, which often is required for thyroid scintigraphy and ultrasound examination, and avoids radiation hazards. Chemical-grade bTSH initially was used for TSH stimulation, but increasing reports of anaphylactoid reactions and the commercial availability of the much safer pharmaceutical-grade rhTSH made the use of bTSH obsolete.[50-52] Initial research on the use of rhTSH in dogs focused on dosage, route, and sampling time points for optimal thyroid function testing.
Initial studies in healthy Beagles showed rhTSH has a biologic effect on the canine thyroid gland and that the IV route allows a maximal increase in TT4 concentrations 4 and 6 hours after injection.[52, 53] The optimal sampling time is considered 6 hours poststimulation.
A prospective study compared the biologic activity of rhTSH and bTSH for TSH stimulation in healthy Beagles. This study concluded that rhTSH and bTSH have equivalent biologic activity and that TSH stimulation with rhTSH (75 μg IV) can be used to confirm euthyroidism.
The value of rhTSH stimulation for the diagnosis of hypothyroidism initially was evaluated in 2 clinical studies.[55, 56] In both studies, TSH stimulation correctly identified all confirmed hypothyroid dogs and a normal response was obtained in most cases of NTI. These studies showed that TSH stimulation with rhTSH is a valuable tool for the assessment of thyroid function in dogs suspected of hypothyroidism when a definitive diagnosis cannot be made based on basal serum TT4 and TSH concentrations alone.
Different studies have investigated the optimal dosage of rhTSH for TSH stimulation. Most studies have shown there is a dose-dependent effect of rhTSH on poststimulation TT4 concentration which seems to be independent of body weight.[52, 57] The most recent study comparing 2 rhTSH dosages (75 μg/dog and 150 μg/dog) in healthy and suspected hypothyroid dogs concluded that use of 150 μg/dog provides higher discriminatory power to differentiate hypothyroidism from NTI. Hence, the authors currently recommend 150 μg/dog for all dogs with comorbidities or those receiving thyroid-suppressing drugs. The only study comparing the value of TSH stimulation and thyroid scintigraphy to differentiate canine hypothyroidism from NTI concluded that thyroid scintigraphy had the highest discriminatory power. However, bTSH and not rhTSH was used in this study.
TSH stimulation with rhTSH also is valuable for the diagnosis of congenital hypothyroidism.2 Repeated administrations of rhTSH (three consecutive days) followed by thyroid scintigraphy help differentiating primary hypothyroidism from central hypothyroidism. This application also had been reported with bTSH.[59, 60]
Regarding interpretation of the TSH stimulation test, the best cut off value to diagnose hypothyroidism is not yet known and different authors have proposed slightly different criteria.[56, 57] Generally, euthyroidism is confirmed when poststimulation TT4 is ≥ 2.5–3.1 μg/dL and hypothyroidism is considered likely when poststimulation TT4 concentration is < 1.6 μg/dL. Interpretation of intermediate results (poststimulation TT4 between 1.6 and 2.5 μg/dL) is difficult and must take into account the clinical signs of the patient and severity of concurrent disease. In 1 study, critically-ill NTI dogs had a blunted response to rhTSH, and it is possible that dogs with comorbidities may exhibit intermediate results.
The major limiting factor for the use of rhTSH in veterinary medicine is its price. Aliquoting and freezing allow clinicians to economize in the use of this hormone and thus to limit costs. Two studies have evaluated the biologic activity of rhTSH after freezing. The 1st study indicated that the biologic activity of rhTSH could be preserved after refrigeration at 4°C for 4 weeks and after freezing at −20°C for 8 weeks. A 2nd study showed that the biologic activity of rhTSH could be preserved for up to 12 weeks after freezing at −20°C. Another shortcoming of TSH stimulation is that it cannot be used to evaluate thyroid function in patients receiving levothyroxine supplementation. Thyroid hormonal supplementation causes thyroid atrophy and must therefore be discontinued before thyroid testing, including TSH stimulation. Although the appropriate period of thyroid hormone withdrawal is unknown, an interruption of 6–8 weeks has been proposed as a guideline.
Optimization of 131I Therapy for Canine Thyroid Carcinoma
Until recently, research on the use of rhTSH in dogs has focused on the diagnosis of hypothyroidism. However, as in humans, rhTSH also may be of value in optimizing 131I therapy for canine thyroid carcinoma. If external beam radiation is not available, 131I may be an excellent treatment alternative for dogs with unresectable or metastatic differentiated thyroid tumors.[63, 64] High therapeutic doses of 131I (555–1850 MBq) usually are required and fatal myelosuppression has been reported as a possible complication after 131I therapy at dosages higher than 160 MBq/kg.[63, 64] Risks of radiation exposure for humans and animals must be kept to a minimum according to the “As Low as Reasonably Achievable” (ALARA) principle. Recently, this group published the results of a prospective study evaluating the effect of rhTSH on the uptake of radioiodine-123 (123I) by the thyroid gland in healthy Beagles. In this study, rhTSH (100 μg/dog IV, 24 or 48 h before 123I) caused no significant change in thyroid radioactive iodine uptake (RAIU). These findings raise important issues regarding the optimal dosage, timing, and route of rhTSH administration when the goal is to increase thyroid RAIU, as earlier studies have shown that bTSH increases thyroid RAIU in healthy dogs and in dogs with secondary hypothyroidism.[66-68] Additional research therefore is warranted.
The use of rhTSH in patients with thyroid carcinoma also raises important safety issues. In humans, rhTSH increases the volume of the thyroid gland in healthy subjects and may cause expansion of primary thyroid tumors and metastases.[69-72] Therefore, to prevent complications associated by compressing adjacent structures, rhTSH should be used carefully in patients with large thyroid tumors or central nervous system, lung, spinal, or bone metastases. In a recent prospective study in healthy Beagles, no significant differences were observed between the effects of rhTSH and placebo on thyroid volume, echogenicity, homogeneity, or capsule delineation. Although these results cannot be extrapolated to dogs with thyroid tumors, it seems unlikely that rhTSH (100 μg/dog IV) would induce a marked increase in canine thyroid gland volume or affect the results of thyroid ultrasonography. Additional studies are still warranted to evaluate the effects of rhTSH on the volume of canine thyroid carcinomas and their metastases.
Use of rhTSH in Cats
Several studies have investigated the use of rhTSH in cats and confirmed its biologic activity in this species. TSH stimulation with rhTSH can be performed in cats by administering 25–200 μg of rhTSH IV and, as in dogs, the optimal sampling point is considered 6 hours postinjection. Given the high cost and equivalent stimulatory effect, most later studies used 25 μg/cat. This is also the dose currently used by the authors for clinical purposes.
Feline thyroid function frequently is assessed with technetium pertechnetate (99mTcO4−) thyroid scintigraphy. This group evaluated the effect of rhTSH (25 μg/cat IV) on thyroid gland scintigraphy in healthy cats and concluded that rhTSH significantly increases thyroid 99mTcO4− uptake. This effect should be taken into account when interpreting thyroid scintigraphy after TSH stimulation.
Feline hyperthyroidism is comparable to human toxic nodular goiter and is the most common endocrine disorder in geriatric cats, with a prevalence of 2%. In most patients, increased serum TT4 concentration is observed, confirming the diagnosis. However, serum TT4 concentration may be within reference range in cats with mild hyperthyroidism (a result of nonspecific fluctuations in thyroid hormone production) or in cats with systemic comorbidities.[78, 79] These challenging cases require further testing such as thyroid scintigraphy, fT4 measurement, T3 suppression test, or TSH stimulation. Thyroid scintigraphy is likely less affected by NTI than basal TT4 and fT4 concentrations, and is the test preferred by the authors to establish a definitive diagnosis in these cases, but there is limited availability, it requires sedation and involves radiation hazards. Although fT4 measurement is highly sensitive, there are some concerns regarding its specificity; 6–21% of euthyroid sick cats may have increased fT4 concentrations, and 20% of euthyroid cats with chronic kidney disease also have increased fT4 concentrations.[79-81] The T3 suppression test is a logical dynamic test for hyperthyroidism but it relies on owner compliance, cat acceptance, and gastrointestinal absorption. Although in euthyroid cats TSH stimulation leads to a marked increase in circulating TT4 concentrations 6 hours after injection, in hyperthyroid cats little change typically is observed as a result of decreased thyroid functional reserve. However, a study evaluating bTSH stimulation in hyperthyroid and healthy cats concluded this test had limited value because hyperthyroid cats with equivocal baseline-TT4 concentrations exhibited a response indistinguishable from that of healthy cats. On the basis of these results and on the fact that stimulation tests generally are not the best choice to confirm hyperfunction of an endocrine organ, the authors do not recommend TSH stimulation in these cases.
131I is considered the treatment of choice for both feline hyperthyroidism and human toxic nodular goiter.[85, 86] As in humans, concerns regarding radioactivity exposure arise in 131I-treated cats. Direct contact with 131I-treated cats and exposure to their excreta can pose a risk to human health. Indeed, 131I-treated hyperthyroid cats excrete a sufficient amount of radioactivity in their urine to require labeling as radioactive material for 21 days after treatment.[88, 89] Moreover, during the 1st week after 131I therapy, the removable radioactivity present on the cat can even exceed the maximum acceptable activity for a noncontrolled area. The remaining radioactivity and the duration of isolation are determined only by the amount of radioactivity administered. It is possible that, as in human toxic MNG, rhTSH may enhance thyroid RAIU in hyperthyroid cats, allowing 131I dose reduction and decreased radiation exposure for cats and their owners. A lower efficacious dose would decrease the surface dose emission rate, the amount of radioactivity found in excreta, hair, and paws, and the required duration of isolation for 131I-treated cats, thereby conforming to the ALARA principle.[88-90] This group recently has evaluated the effects of rhTSH on thyroid RAIU in hyperthyroid cats. In this study, rhTSH administration caused a small but statistically significant increase of 7% in thyroid RAIU. Although the clinical relevance of this increase appears limited, these results are promising and additional studies are needed to investigate the optimal dose and timing of rhTSH injection to optimize 131I treatment of feline hyperthyroidism.
Naturally occurring hypothyroidism is rare in cats. Reported causes include congenital hypothyroidism, lymphocytic thyroiditis, and secondary hypothyroidism after head trauma. TSH stimulation with rhTSH (75 μg IV) has been reported as an aid in the diagnosis of feline congenital hypothyroidism.
Iatrogenic hypothyroidism may be present in 6–30% of cats treated for hyperthyroidism with 131I.[94-96] Occasionally, cats treated with 131I may simultaneously develop azotemia and a serum TT4 concentration below reference range. These patients can present a diagnostic challenge. On the one hand, iatrogenic hypothyroidism could contribute to declining kidney function.[97, 98] On the other hand, the presence of chronic kidney disease (CKD) alone may suffice to suppress serum TT4 to concentrations below the reference range.[81, 99] CKD and iatrogenic hypothyroidism also may be present simultaneously. We recently published a prospective study evaluating TSH stimulation and thyroid 99mTcO4− scintigraphy in healthy cats, cats with NTI, and cats with low TT4 and azotemia after 131I therapy for hyperthyroidism. Serum TT4 concentrations 6 hours postinjection were significantly lower in the cats with low TT4 and azotemia and we concluded that TSH stimulation is useful in differentiating feline iatrogenic hypothyroidism from NTI.
Differences in amino-acid sequences make the β subunit immunogenic in a nonhomolog species. This leads to the risk of antibody induction and hypersensitivity reactions when rhTSH is administered repeatedly. A review of the veterinary literature indicates that at least 547 dogs and 93 cats already have received rhTSH.[52-57, 61, 65, 74-76, 91, 97],3,4 Furthermore, 87 of those 547 dogs and 7 of those 93 cats underwent repeated rhTSH administration. To date, no anaphylactic reactions have been recorded, and the only recorded adverse effect was transient pain after IM injection. Since 2006, approximately 30 dogs have undergone TSH stimulation with rhTSH every year in our clinic. This test also is frequently used in several other clinics around the world. To our knowledge, no serious adverse reactions have been reported. The adverse reactions previously observed with bTSH (ie, anaphylactoid reactions) most likely were a result of the unpurified chemical-grade bTSH rather than its amino-acid sequence. It is therefore not surprising that the use of a purified pharmaceutical-grade rhTSH does not yield similar adverse reactions.
Since rhTSH became commercially available, clinical research proved that it is a safe and valuable test for the diagnosis of canine hypothyroidism. Recent research also suggests it can aid in the diagnosis of feline hyperthyroidism and feline hypothyroidism. With time, improved diagnostic assays using baseline hormone measurement should become available for use in dogs and cats, replacing functional tests such as TSH stimulation and thyroid scintigraphy. In the future, rhTSH could be used in 131I-therapy optimization for the treatment of canine thyroid carcinoma and feline hyperthyroidism.
Genzyme Corporation, Cambridge, ME
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