Pertinent studies are scarce, but they exist
I don't know if the shortage of scientific evidence is a result of the general ignorance of professional athletes, trainers and above all the "average gymrat" that he or she has an organically healthy thyroid that's just shut off by your body in order not to waste precious muscle and organ weight in a state of constant catabolism.
Despite the scarcity of research, there are two pertinent studies from the Department of Biological Sciences at the College of Osteopathic Medicine of the Ohio University. Both studies were published in the early 1990s and deal - as you would expect it with female subjects (with no recent history of dieting or weight loss were recruited from the university and surrounding community) who were randomly assigned to a 3x2 experimental design of aerobic exercise and energy availability treatments. The subjects had kept detailed food logs (including weighing and measuring all their foods) before the intervention to have a baseline reading for their energy intake and their heart rates and VO2 max were established during testing seasons before the first workout day.
Table 1: Detailed information about the participants in the 1993 (Study A) and 1994 (Study B) studies by Loucks et al. (Loucks. 1993 & 1994)
- participants in study B (Loucks. 1994): 28 volunteers 18-29 years of age
- none of the participant was using medication including oral contraceptives, no history of heart, liver, or renal disease, diabetes, menstrual or thyroid disorders, or a history of severe dieting; all had at least 3 mo of documented menstrual cycles 26-32 days in length and had exercised for only ~60 min per week in the previous 3 months
- exercising not at all (Z; zero exercise group),
- expending 1,300kcal/day on low (40% VO2max) intensity exercise (LO)
- expending 1,300kcal/day on high (70% VO2max) intensity exercise (HI)
"So what did happen? Did they drop dead?" Not exactly, no...
- 30kcal/kg body weight per day (~1,750kcal/day of available energy) in the normal energy group (B) and
- 8kcal/kg body weight per day (~500kcal/day of available energy) in the low energy group (D)
The blood was sampled for 8 days beginning on the day before the intervention began, in order to track (a) the time course of the development and (b) the persistence of the hormonal changes in the subjects.
|Figure 2: Thyroid hormones at the end of the 4-day intervention expressed relative to baseline (Loucks. 1993)|
"[w]hen dietary energy intake was increased in exact compensation for the energy cost of exercise (LB, HB) [...] even the large volume of exercise performed in this study (similar to running a half marathon each day) had no effect on T3 levels." (Loucks. 1993)I have my doubts whether "training in the zone" for hours everyday will not shut down the thyroid after 3 weeks and thus 5x the duration of the study. I mean -10% T3 & 5%+ increase in reverse T3 are certainly boding ill (see my comment in figure 2). The data from the two-way ANOVA plotted in figure 3, on the other hand, confirm that the scarcity of energy and thus your bodies' desire to conserve energy is the main factor, here.
"You're thyroid function is fine - rather high than low!", says the Dr. to the patient
The experiment Loucks et al. conducted does also shows why most patients don't receive adequate counseling (there is no need for treatment) by their doctors and statements like the one above are rather the rule than the exception.
|Figure 3: Treatment effects on the changes of total T3 (in nmol/l) in the study particpants of study A (Loucks. 1993)|
In a way this is yet better than a Dr. prescribing T4 in these situations. The latter will only be converted to rT3 and can aggrevate the sluggishness and apathy that comes with the low metabolic function due to "low T3 syndrome".
We need more data: Study B (Loucks. 1994)
|"How did I let this happen again?", asks Oprah in her own magazine - the answer is simple, Oprah! Your "diet" programmed the YoYo effect! It happened not after, but right while you were starving... ah, pardon "dieting"! Learn more about the benefits of dieting down slowly.|
- whether exercise training is capable of altering thyroid metabolism in women, and, if so,
- whether this alteration can be wholly explained by the impact of exercise on energy availability, with all other physiological processes occurring during exercise training (i.e., “exercise stress”) having no influence on thyroid metabolism.
Still, I and I would guess, you, as well, will appreciate that Loucks & Heath conducted a follow up study, in the course of which they wanted to elucidate how important the dietary compensation of the exercise induced energy expenditure is, if you want to keep your thyroid hormone metabolism intact. To this ends the subjects were randomly allocated to four groups (ordered from lowest to highest energy intake):
- 10.8kcal/kg LBM available energy -- this is what's left from a baseline intake of 39.5kcal/kg LBM, after the energy expended during the workouts is subtracted from a diet with a caloric deficit of ~23% below the participants' habitual intake
- 19.0kcal/kg LBM available energy -- this is what's left from a baseline intake of 48.6kcal/kg LBM, after the energy expended during the workouts is subtracted from a diet with a caloric deficit of ~6% below the participants' habitual intake
- 25.0kcal/kg LBM available energy -- this is what's left from a baseline intake of 53.4kcal/kg LBM, after the energy expended during the workouts is subtracted from a diet that had an identical energy content as the participant's habitual diets
- 40.4kcal/kg LBM available energy -- this is what's
left from a baseline intake of 68.4kcal/kg LBM, after the energy
expended during the workouts is subtracted from a diet with a caloric
content that was ~32% above the participants' habitual diets (typo in kcal values corrected)
|Figure 4: Effects of training at 70% VO2max aiming for a total energy expenditure of 30kcal/kg LBM at different levels of available energy (intake - expenditure in kcal/LBM) on thyroid hormones; values expressed rel. to baseline (Loucks. 1994)|
|Figure 5: The changes in thyroid hormones come "stepwise" with thresholds at 22.5kcal/LBM body weight for FT3 and ~14kcal/LBM available energy for FT4 and rT3 (Loucks. 1994)|
As you can see, your body is smarter the average starvation dieter thinks he is. So the bottom line is easily formulated: Don't starve yourself if you don't want do feel and look like miserable, hold water, get fat from whatever you eat and end up as a physiological and psychological wrack.
- Loucks AB, Callister R. Induction and prevention of low-T3 syndrome in exercising women. Am J Physiol. 1993 May;264(5 Pt 2):R924-30.
- Loucks AB, Heath EM. Induction of low-T3 syndrome in exercising women occurs at a threshold of energy availability. Am J Physiol. 1994 Mar;266(3 Pt 2):R817-23.