Over-replacement of thyroxine

Dear Ask The Doctor: I was told I had mild hypothryoidism in March of this year and started taking Levothyroxin 125mcg. In May I had blood work done and the TSH level was .008 so my doctor told me to stop taking the medicine and to have more blood work done in 8 wks to see what the TSH level was then. Is this too long to wait for blood work and should I be off this medicine cold turkey for so long? I was lead to believe once you start this medicine you would be on it for life? Thank you for you help.

Dear Keri: The normal reference range of Thyroid Stimulating Hormone (TSH) is 0.4 – 5.0 mIU/L, however laboratories vary slightly in their reference ranges. TSH does exactly as the name suggests, it stimulates the thyroid gland to produce the hormone thyroxine. If the dose of thyroxine you are taking is adequate for your body's specific requirements, your TSH level should be within the normal range. However your TSH level is well below the lower normal limit and this suggests that your current thyroxine dose is much to high for your body's requirements. It is reasonable practice to discontinue thyroxine and repeat thyroid function tests in a 8 week period, when TSH levels may have returned to within normal range, and an assessment can be made determining the need for future low dose thyroxine replacement.