Desiccated thyroid and presumably other T4 and T3 combinations result in T3-RIA values that may become elevated for several hours after administration and then decrease into the reference range. T4 values remain within the reference range if replacement is adequate. In the few instances when clinical evidence and T4 results disagree, TSH assay is helpful. Elevated TSH values suggest insufficient therapy. Unfortunately, low TSH values using standard TSH kits is not a reliable indicator of over treatment since most of these kits produce considerable overlap between normal persons and those with decreased values in the low reference range area. Ultra sensitive TSH kits should solve this problem if the kit is reliable.

L-Thyroxine (Synthroid, Levothroid) results in T3-RIA values that, in athyrotic persons, are approximately two thirds of those expected at a comparable T4 level when thyroid function is normal. This is due to peripheral tissue conversion of T4 to T3. The T3-RIA values are more labile than T4 values and are more affected by residual thyroid function. The standard test to monitor L-thyroxine therapy is the T4 assay. There is disagreement whether T4 values must be in the upper half of the T4 reference range or whether they can be mildly elevated. In general, when the TSH value returns to its reference range, the T4 level stabilizes somewhere between 2 µg above and below the upper limit of the T4 range. T4 elevation more than 2 µg above reference range probably suggests too much dosage. A minority believe that T4 values should not be above the reference range at all. On the other hand, T4 values in the lower half of the reference range are usually associated with elevated TSH levels and probably represent insufficient replacement dose. Some investigators favor T3-RIA to monitor therapy rather than T4 or TSH. The THBR value is most often within reference range with adequate replacement dose but has not been advocated for monitoring therapeutic effect.

One report indicates that dosage requirement decreases after age 65 years.