ELSEVIER Bone Vol. 16, No. 5 May 1995:505-509 A Histomorphometric Study of Bone Changes in Thyroid Dysfunction in Rats T. J. ALLAIN,I M. R. THOMAS,I A. M. McGREGOR,1 and J. R. SALISBURY 2 Departments of ~Medicine and 2Histopathology, Kings College of Medicine and Dentistry, London, UK Clinical studies in thyrotoxicosis reveal a state of high bone turnover leading, eventually, to osteoporosis. Recently there has been concern that thyroxine (T4) treatment may have a similar effect on bone. Rat models have been used to study the effects of T 4 on bone, but the majority of studies have looked at the effects of T 4 after only 3 weeks of treatment. The aim of this study was to evaluate histomorphometric changes in rats after 12 weeks of thyroxine overtreatment or 12 weeks of hypothyroidism compared with untreated con- trol animals. Animals received either T 4 200 ttg/kg per day, 0.1% propylthiouracil, or vehicle for 12 weeks. Tetracycline was administered 1 week and 3 weeks prior to killing. Iliac crest bone was used for histomorphometry. Serum T 4 mea- surements (taken at killing) confirmed hyper- and hypothy- roidism in the appropriate animal groups (between group difference p < 0.001 by ANOVA). In hyperthyroid animals there was an increase in mineral apposition rate (MAR; 0.94 vs. 0.59 Itm/day, p < 0.001) and mineral formation rate (MFR/BS; 0.24 vs. 0.12 x 10 -2 I.tm3/~m 2 per day, p < 0.001) and a slight increase in eroded surfaces (ES/BS%; 1.54 vs. 1.36, p < 0.05) compared with controls, consistent with previous in vitro and in vivo observations. In hypothy- roid rats there was a marked reduction in osteoid surfaces (OS/BS%; 1.7 vs. 24.8, p < 0.001) and MAR (0.3 vs. 0.59 itg/day, p < 0.001), a reduction in ES/BS% (0.51 vs. 1.36, p < 0.05), and an increase in cancellous bone volume (BV/ TV%; 30.29 vs. 19.6, p < 0.05), suggesting that thyroid hormones are a requirement for normal bone turnover. This study characterizes the histomorphometric changes following 12 weeks of T 4 overtreatment in rats and demonstrates pro- found changes due to hypothyroidism in a rat model. To the best of our knowledge, it is the first study to show such effects in an animal model. (Bone 16:505-509; 1995) Key Words: Rat; Thyroxine; Propylthiouracil; Hyperthy- roidism; Hypothyroidism; Histomorphometry. Introduction The association between thyrotoxicosis and osteoporosis was first described over 100 years ago. 39 However, it was not until the 1970s that it became apparent that thyroid-associated meta- bolic bone disease had distinct histological features. 1~2~ These Address for correspondence and reprints: Dr. J. R. Salisbury, Depart- ment of Histopathology, Kings College School of Medicine and Den- tistry, Bessemer Road, London SE5 9PJ, UK. comprehensive studies have characterized the bone histomorpho- metric changes in thyrotoxic humans. The histological consequences of hypothyroidism in bone are less well documented; there have been fewer studies, patient numbers in these studies have been smaller, and the results less consistent.7'22 With awareness of the possible harmful effects of thyroxine (T4) treatment on bone ~8"28"32 attention has turned to the use of animal models to characterize the effects of thyroxine overtreat- ment. Rat models of hyperthyroidism and hypothyroidism have been used to study effects on bone mineral densityY '26'3~ cal- cium and phosphorus balance, 5'4° bone biochemical mark- ers, 31"42 and mRNA expression for marker proteins in bone. 26"33 Histomorphometric studies of the effects of thyroid dysfunc- tion in rats have documented changes in tibial morphometry after 3 weeks of excessive T4 treatment. No such effect was observed in the vertebrae of the same animals. 3"a2 The complete bone remodeling cycle in rats takes at least 16 days, 36 and it has been suggested that 3 weeks is insufficient time to observe steady- state changes in rat models of metabolic bone disease.12 There have been no reports of histomorphometric changes in animal models of hypothyroidism. This aim of this study was to evaluate histomorphometric changes in rats rendered thyrotoxic for 12 weeks by thyroxine overtreatment or rendered hypothyroid for 12 weeks by propyl- thiouracil treatment to characterize the steady-state effects of thyroxine overtreatment in bone and to determine the morpho- metric consequences of hypothyroidism in an animal model. Method Preparation of the Animal Model Eighteen 6-week-old male Wistar rat littermates, weighing 201- 297 g, were obtained from Interfauna UK Ltd. (Huntingdon, UK). Rats were maintained 3 to 4 per cage, kept in the same room with 12-h light/dark cycles at a temperature of 22°C and were allowed to eat and drink ad libitum. Animals were ran- domly divided into three groups of 6, consisting of a control group, a hyperthyroid group, and a hypothyroid group. Thyroid dysfunction was maintained for a period of 12 weeks. Hyperthyroidism was induced by the addition of thyroxine (1"4) to the drinking water at an estimated total dose of 200 ixg/kg per day per animal. Thyroxine (Sigma Chemical Co. Ltd., Poole, UK) was diluted in 40 parts propylene glycol, 40 parts distilled water, and 1 part diethanolamine, to give a final con- centration of 1 mg/mL. The thyroxine solution was then dis- solved in 1% bovine serum albumin (BSA) and rats allowed to drink ad libitum. © 1995 by Elsevier ScienceInc. 505 8756-3282/95/$9.50 8756-3282(95)00074-N