Hypothyroidism is a condition in which thyroid glands are unable to produce normal amount of hormone called thyroxine. Test like T3 & T4 are usually used to diagnosed the thyroid function. Measuring levels of T3 and T4 alone is not sufficient to diagnose thyroid problem because these can be normal but person can have thyroid symptoms. Thyroid glands secrets thyroxine under the influence of TSH. The levels of TSH become high when thyroid glands are under active. TSH levels are lower than normal if thyroid glands are hyperactive. So, TSH test is more reliable in the diagnosis of thyroid problems than T3 and T4 alone.