Table 4.

Suggested Sequence of Diagnostic Procedures in Suspected RTHß

1.

Usual presentation: high serum levels of free T4 with non-suppressed TSH.

2.

Confirm the elevated serum level of free T4 and exclude interfering substance, such as antibodies to T4, and other serum TH transport defects, especially if T3 is normal and obtain free T4 measurement by equilibrium dialysis

3.

Obtain tests of thyroid function in first-degree relatives; parents, sibs and children

4.

Sequence the THRB gene and if a mutation is detected shown to have an impaired function, the diagnosis of RTHß is secured

5.

In the absence of THRB gene mutation and lack of abnormal thyroid function tests in other family members, the presence of a TSHoma should be excluded by measurement of the α-SU in serum and other appropriate tests (T3-suppression, TRH stimulation and MRI).

6.

Demonstrate a blunted TSH-suppression and metabolic response to the administration of

supraphysiological doses of TH (see response to L-T3 protocol, Figure 5).
7. Blunted TSH response to L-T3 with absence of THRB gene mutation in indicates nonTRß-RTH, which includes possible THRB mosaicism.

From: Impaired Sensitivity to Thyroid Hormone: Defects of Transport, Metabolism, and Action

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