Role of thyroid hormones in human and laboratory animal reproductive health

Preclinical Development of New Drugs that Enhance Thyroid Hormone Metabolism and Clearance: Inadequacy of Using Rats as an Animal Model for Predicting Human Risks in an IND and NDA

New drugs that enhance metabolism or clearance of thyroid hormones in rats often trigger a sequence of toxicity events during chronic administration: reduction of thyroxine, elevation of thyroid-stimulating hormone (TSH) levels, and thyroid gland hyperfunction/growth. Hepatocellular hypertrophy and thyroid follicular hyperplasia are often observed with increased liver and thyroid organ weights. This unique toxicity profile seems to be species-specific because the thyroxine in rodents is metabolized rapidly, without thyroid hormone-binding globulin that serves as a reserve, as in humans. Thus, elevations of TSH were not reported in humans for drugs such as delavirdine, fluvastatin, nicardipine, phenobarbital, simvastatin, and spironolactone, all of which produce thyroid hyperplasia or tumors in rats. Further, the human thyroid is less sensitive to prolonged TSH stimulation than that of the rat (eg, endemic goiter patients with high TSH due to iodine deficiency do not develop thyroid cancer). In view of the species difference in sensitivity of the thyroid between rodents and humans, using the rat as an animal model to explore target organs of toxicity for a new drug that significantly enhances thyroid hormone metabolism/clearance and increases TSH levels would not be adequate. In this case, a compromised and dysfunctional hypothalamo-pituitary-thyroid system would confound the toxicity profile explored in preclinical toxicity testing and render the model an inadequate risk predictor for the new drug in humans. Under such conditions, IND and NDA sponsors of drugs exhibiting this activity profile should be encouraged to use alternative animal species for toxicity exploration to provide a more meaningful human risk prediction.

Chronic Estradiol Exposure Modulates Thyroid Structure and Decreases T4 and T3 Serum Levels in Middle-Aged Female Rats

OBJECTIVES

In human medicine, estrogen is applied in prevention and treatment of health problems associated with the menopause. The aim of this study was to examine the effects of chronic estradiol dipropionate (EDP) treatment on thyroid gland structure and function in middle-aged female rats.

METHODS

At 14 months of age, Wistar rats received 0.625 mg EDP/kg b.w./day intraperitoneally for 2 weeks. The peripheral and central zones of the thyroid were stereologically analyzed and the following morphometric parameters determined: volume density of follicles, follicular epithelium, interstitium and colloid, epithelial height and the index of activation rate. Serum levels of TSH, T4 and T3 were determined by ELISA.

RESULTS

EDP treatment led to significant decreases in volume densities of follicles and follicular epithelium, epithelial height and index of activation rate (by 11%, p < 0.05; 23%, p < 0.005; 11%, p < 0.05 and 21%, p < 0.05, respectively) in comparison to control values. Hyperplasia of thyroid follicular cells was noticed in 25% of EDP-treated animals. Serum levels of T4 and T3 were decreased (by 33%, p < 0.005 and 28%, p < 0.001, respectively), but TSH concentration was not significantly different from that of the controls.

CONCLUSION

Chronic estradiol treatment significantly decreased volume density and height of centrally located follicular epithelium, follicular activation index and serum level of total thyroid hormones in middle-aged rats.

Center for the Evaluation of Risks to Human Reproduction?The First Five Years

The National Toxicology Program (NTP) Center for the Evaluation of Risks to Human Reproduction (NTP-CERHR) was established by the NTP and the National Institute of Environmental Health Sciences (NIEHS) in 1998 to address the impact of chemical exposures on human reproductive and developmental health and to serve as an environmental and reproductive health resource for government agencies and the general public. The purpose of this report is to provide an overview of the Center activities and a summary of NTP conclusions on chemicals evaluated during this time period. CERHR evaluations involve the critical review of reproductive, developmental, and other relevant toxicity data by independent panels of scientists. The products of these evaluations are expert panel reports. The public has opportunities to provide oral comments at the panel meeting and written comments on draft and final expert panel reports. The NTP evaluates these comments, the conclusions of the expert panel, and any new data not available at the time of the panel meeting, and prepares an NTP brief that describes in plain language the NTP's conclusions on the reproductive and developmental hazard from specified chemical exposures. The NTP brief, expert panel report, and public comments comprise the NTP monograph on the chemical. Monographs are sent to federal regulatory agencies, the NTP Executive Committee, and the NTP Board of Scientific Counselors, and are publicly available. Over the last five years, CERHR conducted expert panel evaluations on 14 chemicals. At this time, 13 panel reports have been published and 12 NTP-CERHR monographs have been issued. Additionally, CERHR conducted a 2-day workshop on the role of thyroid hormones in reproductive and developmental health.