I think Peat would interpret this study as what are the effects of not treating thyroid dysfunction.
They used T4 alone. Peat often refers to that treatment as anti-thyroid, or thyroid suppressive. So all you can conclude from the study is what are the effects of mistreating hypothyroidism.
A hypothyroid person produces lesser amounts of both T3 and T4, so to replace what the glands aren't producing you need to supplement both T4 and T3. Supplementing only T4 might reduce the overall production of hormones in the gland, which can result in even lower levels of T3, leaving the person more hypothyroid than before treatment.