My TSH has also been extremely low since starting Cytomel. My T3 and T4 are fine, and have been for years. Actually it started when I switched from Synthroid to Armour Thyroid, then to Thyrolar. I feel too much of a difference with the additional T3 and do not want to go back to only taking Synthroid. I tried cutting back the Synthroid as recommended by the endocrinologist I was seeing. She refused to believe that it was the Cytomel, and just kept insisting I was hyperthyroid. My T3 and T4 continued to drop, but she said that "only the TSH counts"! When I was back to sleeping 14 hours at night and still falling asleep at my desk, I quit seeing her and went back to my regularly scheduled dosage of Levothyroxin and Cytomel.
So now I wonder, if it is the additional T3 that causes the TSH to "go missing" and what it really means?
The amount of cytomel is fairly high (total of 40mcg/day -- roughly the equivalent of 160mcg of synthroid, which in addition to the 125mcg 1/2 tab is 222.5mcg of T4/day -- more than what most people need). Going through menopause may be more of the problem than a thyroid imbalance but change in estrogen status may alter thyroid requirements. Would consider consultation with an endocrinologist to sort out the best options.