In 2002 National Academy of Clinical Biochemists put out a position paper suggesting it should be .3-2.5 and in 2003 the American Association of Clinical Endocrinologists said the range should be .3-3.0. In actuality it still varies all over the place with the top somewhere
between 4.0-5.5. This video explains the science behind NACB's suggested 2.5. It is from the 2003 meeting of the American Thyroid Association. It is about 18 minutes and presents one of the most important concepts that most doctors don't know about thyroid blood tests, a low index of individuality. Click on Dr. Lee's "TSH Reference Range Redefined: What does it Mean and What Should We Do?" The others are interesting if you have the time. PR
This is on the history of thyroid hormone treatment over the last 100 years. The slides do not work but the audio does. It is a good overview.