From Mary Shomon,
Your Guide to Thyroid Disease.
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People frequently have different TSH levels from blood test to blood test. What factors might account for a TSH that is changing? Are there things you may be doing that are affecting your TSH? Let's take a look at some of the factors that can cause your TSH to fluctuate.

1. Change in Dosage -- changing your thyroid medication may change your TSH.
2. Potency Fluctuations in your medication
3. Lab changes or mix-ups
4. The timing of when you take your pill
5. Starting or stopping a high-fiber diet
6. Starting/stopping calcium or iron supplements
7. Eating too many soy products
8. Eating too many goitrogenic foods, which include Brussels sprouts, rutabaga, turnips, kohlrabi, radishes, cauliflower, African cassava, millet, babassu (a palm-tree coconut fruit popular in Brazil and Africa) cabbage, and kale
9. Change of seasons -- TSH can rise in the colder months
10. Hormonal fluctuations
11. Pregnancy
12. Certain herbs/Supplements
13. Certain prescription drugs
14. Stress or illness
15. Progression of your thyroid disease.

More detailed information on why your TSH might be fluctuating and how to prevent fluctuation is featured in this online guide.