Description

The study will be conducted in three stages.

Stage 1 ( Patient identification and enrollment)

We will generate a list of all SHT patients from the HNP database and all patients will be contacted by the principal investigator (or their designate) regarding the study. Those patients who agree to participate in the study will be given a written consent form. We aim to enrol approximately 90 -100 patients for the study. Laboratory samples for serum TSH and T4 levels will be collected to confirm that all patients are adequately replaced for SHT. Those patients who are not adequately replaced and have low T4 will be excluded from the study.

Stage 2 (T4 withdrawal protocol)

Stepwise T4 replacement therapy will be initiated as follows.

Step 1:

• Reduce the dose of T4 therapy by 50% and recheck serum T4 after 2 weeks.
• If serum T4 is normal, go to step 2.
• If serum T4 drops to below normal then resume full T4 replacement (these patients will be considered as having withdrawal failure).

The rationale for testing serum T4 after 2 weeks is that the half life of T4 is up to 7 days and early testing will potentially give false results. Furthermore, serum T3 is not a reliable test in patients taking T4 therapy and will not be requested 6.

Step 2:

• Discontinue T4 therapy altogether and recheck serum TSH and T4 after 2 weeks.
• If serum T4 is normal, go to step 3.
• If serum T4 drops to below normal then resume full T4 replacement (these patients will be considered as having withdrawal failure).

Step 3:

• Recheck serum TSH and T4 after 8 weeks.
• If serum TSH and T4 levels are normal, these patients will be regarded as having successful T4 withdrawal.
• If T4 drops to below normal then resume full T4 replacement (these patients will be considered as having withdrawal failure).

Stage 3 (Analysis)