Endocrinology

6 Endocrinology




FREQUENTLY ASKED QUESTIONS



1) Is it dangerous to give thyroid hormone to a show dog?

All dogs should be tested before ever being given thyroid hormone and should get thyroid supplement only if they are diagnosed as hypothyroid with accurate testing (measurement of free thyroxine [T4] by the dialysis method and measurement of thyroid-stimulating hormone [TSH]). I have had people tell me that administering thyroid hormone gives their dog a thick coat and makes it more “showy”; I would probably jump around more if I were hyperthyroid, too! Hyperthyroidism (too much thyroid hormone) is detrimental to the animal. Giving thyroid hormone supplement shuts down normal thyroid hormone regulation, making the dog hypothyroid when you stop giving the supplement. Hypothyroidism (too little thyroid hormone) is also detrimental to the animal. I know you can get thyroid hormone over the Internet, but I strongly discourage you from doing so. Test the animal first, use accurate tests, and supplement only if necessary.


2) Do human early pregnancy tests (EPTs) work in dogs?

No. The hormone measured in human EPTs is human chorionic gonadotropin, which is produced only in humans. Dogs do not produce a similar hormone early in pregnancy.


Endocrinology is the study of hormones. It is a huge subject, and a comprehensive review is well beyond the scope of this book. Some generalities of endocrinology and the glands and hormones of interest to small animal reproduction are described, however. The reader must access other sources of information for complete details about other aspects of endocrinology.


Hormones are chemical substances secreted from glands through ducts and sent to other tissues through the bloodstream or secreted by glands with no ducts into the surrounding tissue. These chemicals affect only certain tissues and therefore evoke a specific response. Tissues cannot respond to a hormone unless that tissue contains receptors for that hormone. Receptors act like gates on the surface of the cell, with the hormone acting like a key; if the key fits the gate, a response occurs within that cell (Figure 6-1).


image

Figure 6-1 Hormone/receptor interaction on the cell.


Hormones have one of two chemical structures. Some hormones are proteins. These hormones, like all proteins, are composed of amino acids joined in a specific sequence. The exact chemical structure of protein hormones differs between species; human insulin is not identical, chemically, to canine insulin. Because protein hormones differ between species, administration of a hormone from one species into an individual of another species will not always cause the expected effect. Also, measurement of protein hormones is more difficult because the assay, or test used to measure the concentration of the hormone in the bloodstream, must have been tested and proven accurate for that species.


The other class of hormones is steroids. The steroid hormones—testosterone, estrogen, progesterone, and cortisol—all have a very similar chemical structure (Figure 6-2). Steroid hormones do not differ between species, so they can be used interchangeably for treatment and measured in the blood of any species using any species’ assay.


image

Figure 6-2 Molecular structure of steroid hormones.


Hormone secretion is not constant. A physiologic or neurologic stimulus either stimulates or prevents release of a given hormone from its site of production or storage. Positive stimuli cause release of a hormone; negative stimuli prevent its release. Physiologic stimuli that may cause hormone release include changing hormone concentrations in the body and changes in other physiologic parameters in the body, such as blood pressure or concentration of sugar in the blood. Neurologic stimuli include physical stimulation, such as suckling puppies, and environmental changes, such as an increase in day length.


An important concept in the control of hormone secretion is feedback. Increasing concentrations of a given hormone in the bloodstream will usually “feed back” to the stimulus that caused release of the hormone, thereby stopping further production. An example is testosterone secretion. The release of interstitial cell–stimulating hormone from the pituitary gland stimulates the release of testosterone from the testes. Increasing concentrations of testosterone in the bloodstream signal the pituitary to stop release of interstitial cell–stimulating hormone, which in turns stops further production of testosterone. This is called a negative feedback loop. When concentrations of testosterone in serum fall, the pituitary is again stimulated to release interstitial cell–stimulating hormone and the cycle starts again. This is an important concept because it alters how we use hormones for therapy. Any hormone introduced into the body may exert negative or positive feedback on glands and tissues, altering the animal’s ability to produce or secrete that hormone now and in the future. Caution must be employed when introducing hormones into the body.



I. ENDOCRINE TISSUES AND THEIR PRODUCTS



A. HYPOTHALAMUS


The hypothalamus lies at the base of the brain (Figure 6-3). It has a unique blood system connecting it to the pituitary gland, which lies beneath it. This blood system, the hypophysial portal system, carries hormones from the hypothalamus directly to the pituitary without circulating it elsewhere in the body. This means that only very tiny concentrations of hypothalamic hormones need be produced to stimulate an effect in the pituitary gland.


image

Figure 6-3 Endocrine tissues and their products.


All hormones produced and released from the hypothalamus cause a direct positive or negative effect on the secretion of hormones from the pituitary. The hypothalamic hormone of greatest interest to small animal reproduction is gonadotropin-releasing hormone (GnRH), a very small protein hormone. The stimuli that cause release of GnRH are not well defined but probably include environmental stimuli and feedback from secretion of pituitary hormones. GnRH causes release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) from the pituitary. GnRH is used clinically to induce ovulation (release of eggs from the ovary), and related compounds have been described for inducing estrus in bitches and, with long-term therapy, for suppressing estrus. It has been described as a reversible contraceptive in males in some species. GnRH also can be used in males to cause release of LH, which in turn will cause release of testosterone. This is useful if there is a need to measure testosterone production in a dog and can be used occasionally to increase libido in male dogs.


One of the other protein hormones released from the hypothalamus is thyrotropin-releasing hormone (TRH). Release of TRH occurs when blood concentrations of thyroid hormones fall. TRH causes release of thyroid-stimulating hormone (TSH) from the pituitary. There are no clinical uses for TRH, and it is not available as a commercial product.



B. PITUITARY


The pituitary is a bilobed gland at the base of the brain. The two lobes are the posterior lobe, sometimes called the neurohypophysis, and the anterior lobe, or adenohypophysis. The two lobes arise from different tissues during embryologic development. There are no hormone-producing cells in the neurohypophysis. Rather, nerve cells that lie in the hypothalamus produce hormones that are stored in the posterior lobe. The anterior lobe is a true gland; hormones are produced and stored in the anterior lobe.


The hormones of the posterior lobe are oxytocin and vasopressin. Vasopressin is involved in kidney function and will not be discussed here. Oxytocin is a protein hormone. It is released secondary to neurologic stimulation. The two stimuli best documented to cause oxytocin release are pressure of the head of a puppy into the cervix during whelping (Ferguson’s reflex) and suckling of the mammary glands by the pups. In both instances, nerve cells are stimulated to fire, including those in the hypothalamus that produce oxytocin. Stimulation of these nerves causes production of oxytocin and release of the hormone from its site of storage in the posterior pituitary. Oxytocin causes synchronized contractions of the uterus during parturition and contributes to further dilation of the cervix. Oxytocin receptors are not always present on the uterus. Stimulation of the uterus with another hormone, estrogen, increases the number of oxytocin receptors. For this reason, oxytocin is not used in some disease conditions that require contractions of the uterus to empty it of discharge. Oxytocin can cause very strong uterine contractions if given at the right time and should always be used with care and only on the advice of a veterinarian. Clinical use of oxytocin during whelping is discussed in Chapter 12.


During lactation, oxytocin stimulates letdown of milk into the mammary glands. Oxytocin does not stimulate milk production, only milk letdown. Lack of milk production (agalactia) is uncommon in dogs (see Chapter 18). If oxytocin is going to induce milk letdown, it will work within seconds of intranasal or systemic (injected) administration.


The anterior lobe of the pituitary produces and stores the majority of the hormones in the body. For this reason, the pituitary is sometimes called “the master gland of the body.” Hormones secreted from the anterior pituitary include FSH; LH, which also is called interstitial cell stimulating hormone; prolactin; TSH; and adrenocorticotropic hormone (ACTH).


FSH is a large protein hormone secreted both in males and females. It is named for its role in the female, in which it stimulates maturation of ovarian follicles in preparation for ovulation (see Chapter 7). In males, FSH stimulates production of a protein in the testes that binds testosterone tightly within the testis to promote spermatogenesis. FSH is not available as a commercial product and has no clinical use at this time.


Another large protein, LH, also is named for what it does in the female. Luteinization in dogs occurs in the ovarian follicles around the time of ovulation (see Chapter 9). LH could be given to dogs to induce ovulation but is not commercially available. A human hormone produced during pregnancy, human chorionic gonadotropin (hCG), when administered to dogs, binds to LH receptors and can be used to induce ovulation. LH is also called interstitial cell stimulating hormone, named for its function in males. In male dogs, LH stimulates release of testosterone from the testes, which is necessary for continuing production of spermatozoa. LH production is controlled by a feedback loop with testosterone.


Prolactin is a protein hormone that induces milk production and stimulates normal maternal behavior. Its production is stimulated during pregnancy, and concentrations of prolactin in blood rise sharply at whelping as other hormone concentrations abruptly decrease. Prolactin is not available commercially and has no clinical use at this time.


TSH stimulates production of the two thyroid hormones, triiodothyronine (T3) and thyroxine (T4), described in the next section. TSH is measured as an indicator of hypothyroidism in dogs.


ACTH stimulates the cortex of the adrenal gland to produce steroid hormones, including cortisol, and mineralocorticoids, which control movement of minerals through the kidney. ACTH is measured as an indicator of adrenal disease in dogs.

< div class='tao-gold-member'>

Related posts:

  1. Estrus Suppression, Sterilization, and Pregnancy Termination
  2. Disorders of the Mammary Glands
  3. Infertility
  4. Pharmacology

Stay updated, free articles. Join our Telegram channel
Tags:
Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Endocrinology

Full access? Get Clinical Tree
Get Clinical Tree app for offline access