Fasting or Restrictive Diets. Calorie and carbohydrate
restriction may substantially reduce thyroid hormone
activity. There is a wide range of variation between individuals;
genetics, obesity, gender, and the macronutrient content
of the hypocaloric diet influence the response. Nutritional
status and energy expenditure both influence thyroid function
centrally at the level of TSH secretion, deiodination,
and possibly elsewhere. Because an increase of rT3 is found
at the expense of T3 during caloric restriction, it is possible
that the hepatic pathways playa substantial role in metabolic
control during energy balance. However, when caloric
restriction is longer than three weeks, T4 and rT3 levels
return to normal values.
Fasting also exerts a powerful influence on the metabolism
of thyroid hormones. Mild elevations in endogenous
cortisol levels might be partly responsible. Ketones generated
from calorie deprivation do not appear to suppress T3
generation and hepatic 5'-deiodinase activity. However, it is
not clear whether ketones ha e a similar effect in a caloriesufficient
diet. On a low-calorie diet, elimination of rT3 by
5'-deiodination is decreased. Calories and energy balance
might also influence thyroid hormone metabolism during
increased caloric consumption, during which the clearance
of rT3 by 5'-deiodination is actually increased. On a lowcalorie
diet, elimination of rT3 by 5'-deiodination is
decreased; however, the clearance of rT3 by 5'-deiodination
is actually increased with a high calorie diet.