First of all, are you trying for pregnancy ? Is their any weight  gain ?? Is there any other symptoms . Please read very carefully the following questions and than answer number wise. This will help me reach a proper diagnosis for you. Do you have any of the following symptoms ? 1. Fatigue, loss of energy, lethargy. 2. Weight gain. 3. Decreased appetite. 4. Cold intolerance. 5. Dry skin. 6. Hair loss. 7. Muscle pain, joint pain, weakness in the extremities. 8.Emotional lability, mental impairment. 9.Forgetfulness, impaired memory, inability to concentrate. 10. Sleepiness. 11.Constipation. 12. Fullness in the throat, hoarseness

Weight of pt missing. Whether Tsh 4.2 is first thyroid report onwhich treatment was started .or u always have been taking 100ug and on 100 ug ,ur Tsh is 4.2 then it is little surprising as it is as gud as normal. Only during pregnancy and prepregnancy state or in hyperprolactinemia I this Tsh range, pt is treated. But Tsh 0.1 is too supressed so reduce it by 25 ug and repeat Tsh after 2months

Ar u symptoms free now u can continue same dose.