Effects of phenobarbital on hypothalamic-pituitary-thyroid axis in the rat

Influence of medications on thyroid function in dogs: An update

Erroneous thyroid function test results can occur because of drugs that alter thyroid hormone physiology in one or more aspects, including synthesis, secretion, distribution, and metabolism. Research since publication of the last review in the Journal of Veterinary Internal Medicine (JVIM) 20 years ago has evaluated the effects of amiodarone, zonisamide, inhalant anesthetics, clomipramine, trilostane, and toceranib on thyroid function tests in the dog. In addition, recent work on the effects of glucocorticoids, sulfonamides, phenobarbital, and nonsteroidal anti-inflammatory drugs will be reviewed. Awareness of these effects is necessary to avoid misdiagnosis of hypothyroidism and unnecessary treatment.

Hypothyroidism caused by phenobarbital affects patterns of estrous cyclicity in rats

We found that repeated treatment with phenobarbital (PB), a thyroid modulator, resulted in a persistent estrous stage in the present study. Although the effects of PB in blocking the surge release of luteinizing hormone (LH), inducing anovulation and prolonging the diestrous period has been well established, there is still no research describing the appearance of persistent estrous states in normal cycling rats dosed with PB. To further study this phenomenon, female rats exhibiting regular estrous cycle were administered an oral dose of PB for 14 consecutive days. Consecutively, vaginal smears were observed and rats from all the groups were sacrificed and serum hormone levels for prolactin, progesterone, estradiol, triiodothyronin (T3), thyroxine (T4) and thyroid stimulating hormone (TSH) were measured. Pituitary, thyroid, liver, uteri and ovaries were excised, weighed and further subjected to histological observations. We found that PB induced irregular estrous cycles, especially persistent estrus in rats. Histopathologically, the persistent estrous stages are characterized by persistent vaginal cornification in the vagina, cystic follicles and anovulation in the ovaries. Endocrinologically, serum T3 and T4 levels were significantly lower, and TSH was higher in treated-female rats compared to control females. The serum estradiol level and the estradiol/progesterone ratio tend to increase in treated-females. Furthermore, PB-treated animals with irregular estrous cycle were reduced by T4 replacement. Our data indicate that treatment with PB resulted in hypothyroidism and irregular estrous cycle, particularly a persistent estrous stage in normal cycling female rats.

Influence of drugs on thyroid function in dogs

Several drugs can affect thyroid function test results in humans and eventually lead to an erroneous evaluation of thyroid function. These medications can alter the synthesis, secretion, transport, or metabolism of thyroid hormones. Some drugs also directly inhibit the hypothalamic-pituitary-thyroid axis. The effects of drugs on thyroid function in dogs have long been underestimated and have most likely contributed to the overdiagnosis of hypothyroidism in this species. This manuscript 1st reviews pertinent thyroid physiology followed by an overview of drugs for which the effects on canine thyroid function have been studied. The effects of glucocorticoids, propranolol, sulfonamides, phenobarbital, potassium bromide, and nonsteroidal anti-inflammatory agents (NSAIDs) on canine thyroid function are summarized here. Knowledge of the potential effect of these medications on thyroid function should contribute to a more reliable interpretation of thyroid function test results in dogs.

Prenatal and neonatal pharmacologic stress on early behavior and sexual maturation in the hamster

Exposure to phenobarbital in utero results in sex-dependent altered neonatal behavior and reproductive maturation in hamsters. Prenatal phenobarbital-exposed (40 mg/kg subcutaneous to pregnant dams) male pups have at least 30% less pivoting activity and ultrasonic calling (high-frequency sounds produced by pups when separated from the nest) when compared with control pups born to saline-injected dams. As prenatal phenobarbital-treated male and female hamsters mature, there is a dose-dependent (20, 40, and 60 mg/kg) 2- to 5-day delay, respectively, according to sex, in puberty onset. Both sexes exhibited 30% lighter than control's adrenal weights associated with prenatal phenobarbital exposure. Although gonadal weights for males exposed to phenobarbital in utero were not affected, females had up to 33% lighter ovarian weights when compared with controls. Females exposed to phenobarbital in utero also had approximately 40% more irregular estrous cycles. Gonadal hormones act by means of Central Releasing Factor on pituitary ACTH secretion. Additional treatment during the neonatal period with 10 IU ACTH decreased the percentage of irregular estrous cycles and shortened the delay in puberty onset to near-control values in prenatally phenobarbital-treated females.