INTRODUCTION Hyperthyroidism is associated with a reduction in bone mineral density (BMD). Suppressive doses of thyroxine (T4), inducing subclinical hyperthyroidism, have been reported by some investigators to reduce BMD. Little work has been done on replacement doses of T4. AIM The aim was to investigate the effect of replacement doses of T4 on BMD. STUDY DESIGN Cross‐sectional study of hypothyroid patients on long‐term T4 replacement doses, comparing those who had primary hypothyroidism with those who were previously hyperthyroid. PATIENTS Fifty women on replacement doses of T4 for more than 5 years were recruited. Twenty‐five were treated for primary (group 1) and 25 for radioiodine‐induced hypothyroidism (group 2). They were well matched for age, menstrual status, smoking history, body mass index (BMI), dose and duration of T4 replacement as well as thyroid status. MEASUREMENTS BMD was assessed by dual energy X‐ray absorptiometry. Free T4 (FT4), FT3 as well as ultrasensitive TSH assays were used to assess thyroid status. RESULTS The two groups showed no difference in BMD (g/cm2) of the lumbar spine (1.008 vs. 0.957, P = 0.25), femoral neck (0.745 vs. 0.735, P = 0.79) and total hip (0.878 vs. 0.837, P = 0.24). When the two groups were pooled, there was no significant difference between the patients and a reference population with femoral neck and total hip BMD expressed as a standard deviation (Z) score. However, the lumbar spine mean Z score was significantly greater than zero. For each site, there was a negative correlation of BMD with age in at least one group but, in general, BMI, FT4, FT3 and duration of T4 replacement did not correlate with BMD. T4 dose, however, had a consistent positive correlation with BMD in the spine, femoral neck and the hip (P = 0.01, 0.04 and 0.02, respectively) in group 2 but not group 1. CONCLUSION In this study, there is no evidence for a difference in bone mineral density in patients receiving replacement doses of thyroxine irrespective of the aetiology of their hypothyroidism. The reduced bone mineral density associated with hyperthyroidism appears to be restored, maintained and in some cases possibly improved while on long‐term thyroxine replacement post‐radioiodine.
This paper describes the benefits of integrating optimization formulations within simulation models. Two different case studies in mining are presented, both requiring a blending optimization. The primary problem at hand is to model a complex supply chain involving blending of multiple inputs to produce a number of potential products for customers. The first approach involves solving an optimization model to produce a long term plan, then simulating this plan over time without the ability to change the plan as time progresses. The second approach involves a more integrated system where multiple instances of an optimization model are run throughout the simulation using updated inputs. A description of the problem is supplied, providing the need for both optimization and simulation, and then the two case studies are compared to show the benefits of integrating the optimization within the simulation model. 1898 978-1-4244-9865-9/10/$26.00 ©2010 IEEE
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