I got this from Luura at Ancestralize Me and this is the exec summery. Click the link for more info.

Bottom line: In a previous experiment (12), the scientists had been able to show that energy availability, rather than dietary energy intake or exercise energy expenditure separately, is the behavioral factor affecting thyroid regulation in exercising women. It is thus not exactly surprising that 4 days on an energy deficient diet in the study at hand were enough to induce a low T3 syndrome in the female participants of the study at hand.

Figure 3: Mean T3 thyroid hormone levels after 20 days of total fasting, 800kcal diet without carbohydrates and 800kcal diet consisting almost exclusively of carbs (Spaulding. 1976)

In this context it’s important to point out that the effect occured only, when the dietary inake fell below 50% of the dietary requirement and that the changes in thyroid hormone levels are restricted to T3 and won’t show up on tests that evaluate TSH and T4, only.

via Thyroid Issues? Low Energy Intake Triggers Low T3 / High rT3 Syndrome in Exercising Women >19kcal/kg LBM Avail. Energy Required. Low Carbing Worsens the Impact of ED – SuppVersity: Nutrition and Exercise Science for Everyone.