The Significance of Partial Suppressibility of Serum Thyroxine by Triidothyronine Administration in Euthyroid Man

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Abstract
Recent evidence indicates that triiodothyronine (T3) administration may not completely inhibit normal thyroid secretion. To further corroborate this observation, measurement of serum T4−RIA concentrations was performed on 15 normal controls (10 men, 5 women; ages 20–42) who were placed on 100 μg of T3 daily for a 5–week period. Decrements of 53%, 36%, and 28% from the baseline T4-RIA were noted at weeks 1, 2, and 3 respectively. At 3 weeks, a nadir T4-RIA of 2.5 μg/100 ml was reached which did not significantly differ from the 4th (2.9 μg/100 ml) and 5th weeks (2.6 μg/100 ml). Further, seven euthyroid patients who had received replacement thyroid hormone for 1–16 yr were switched to T3 (75–100 μg/day) for 28 days. At the end of this period, their mean T4-RIA was 2.6 μg/100 ml. Similar T3 treatment studies were performed on 20 primary hypothyroid patients. After 4 weeks of T3, all 20 patients displayed a T4-RIA below the limits of assay detectability (4-RIA with T3 was also noted in 4 patients with pituitary and 2 patients with hypothalamic hypothyroidism. Three days after cessation of T3 treatment in normal subjects, no significant rise in mean T4-RIA was seen (2.3 μg/100 ml). Subsequently, T4-RIA rose to 4.5 μg/100 ml on day 7 and 6.7 μg/100 ml on day 10 (74% of the presuppression value) in normals. A similar rise to 7.9 μg/100 ml 10 days after withdrawal from T3 was noted in the euthyroid subjects who had received long-term thyroid hormone replacement. In contrast, all primary hypothyroid patients had either a minimal or nondetectable elevation in T4-RIA while demonstrating a marked rise in TSH 10 days after T3 withdrawal. An absent or impaired rise in T4-RIA after T3 withdrawal was also noted in patients with pituitary and hypothalamic hypothyroidism. These observations indicated: 1) There is continued thyroidal T4 secretion in euthyroid subjects receiving 100 μg of T3 daily. 2) The hypothesis is advanced that an intact hypothalamic-pituitary-thyroid axis may be required for continued T4 secretion while on T3. 3) The duration of prior suppression with thyroid hormone medication does not appear to influence this phenomenon.

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