Thyroid Function During the Rebound Phase Following the Discontinuation of Antithyroid Drugs

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Abstract
Rats were fed PTU [propylthiouracil] in a low -iodine diet (LID) with or without iodine supplementation for 7 days and thyroid function was assessed during the following 11 days. A peak rebound of I131 uptake and maximum thyroid weight was found at 4 -5 days after drug withdrawal. The biosynthesis of labeled amino acids, as it resumed after the PTU block dissipated, was characterized by very high MIT*/DIT* [monoiodotyrosine/diiodotyrosine] and T3*/T4* [triiodothyronine/thyroxine] ratios. All indices of thyroid function tended to return to the control level between the 6th and 1 lth day after PTU withdrawal. This was more marked in rats fed the iodine-supplemented LID. LID per se increased MIT*/DIT* and T3VT4* ratios and greatly accentuated the shift in these ratios brought about by PTU. If perchlorate was substituted for PTU there was no overshoot of uptake after drug withdrawal and all changes of thyroid function appeared to be those of enhanced iodine deficiency. The most marked single alteration of biosynthesis in PTU and perchlorate withdrawn rats (as well as in LID-fed rats) was the increased synthesis of T3*. The prerequisites for preferential T3* synthesis seem to be marked TSH stimulation and lack of iodine at the substrate level rather than increased MIT*/DIT* ratios.