I found an interesting article that give some light on the question of
low carb and thyroid. The duration of the study was only 8 days. They
found that a lower carb, high fat diet is preferable to lower carb,
high protein diet with respect to blood sugar etc (no big news here).
Note that their diets were not truly "low carb". If I was their
subject, I would eat 210 grams of carbs, which is a lot for me. Even
when I was "eating less" and not low carbing, I would eat only 150
carbs per day.
What's relevant to your question is that they found that low carb
diets led to decrease in T3 concentrations, but not in T4
concentration. Significance of that was unknown to these researchers.
``Effect of low-carbohydrate diets high in either fat or protein on
thyroid function, plasma insulin, glucose, and triglycerides in
healthy young adults.
Ullrich IH, Peters PJ, Albrink MJ.
A low-carbohydrate diet, frequently used for treatment of reactive
hypoglycemia, hypertriglyceridemia, and obesity may affect thyroid
function. We studied the effects of replacing the deleted carbohydrate
with either fat or protein in seven healthy young adults. Subjects
were randomly assigned to receive seven days of each of two isocaloric
liquid-formula, low-carbohydrate diets consecutively. One diet was
high in polyunsaturated fat (HF), with 10%, 55%, and 35% of total
calories derived from protein, fat, and carbohydrate,
respectively. The other was high in protein (HP) with 35%, 30%, and
35% of total calories derived from protein, fat, and
carbohydrate. Fasting blood samples were obtained at baseline and on
day 8 of each diet. A meal tolerance test representative of each diet
was given on day 7. The triiodothyronine (T3) declined more (P less
than .05) following the HF diet than the HP diet (baseline 198
micrograms/dl, HP 138, HF 113). Thyroxine (T4) and reverse T3 (rT3)
did not change significantly. Thyroid-stimulating hormone (TSH)
declined equally after both diets. The insulin level was significantly
higher 30 minutes after the HP meal (148 microU/ml) than after the HF
meal (90 microU/ml). The two-hour glucose level for the HP meal was
less, 85 mg/dl, than after the HF meal (103 mg/dl). Serum
triglycerides decreased more after the HF diet (HF 52 mg/dl, HP 67
mg/dl). Apparent benefits of replacing carbohydrate with
polyunsaturated fat rather than protein are less insulin response and
less postpeak decrease in blood glucose and lower triglycerides. The
significance of the lower T3 level is unknown.
Publication Types:
Clinical Trial
Randomized Controlled Trial
PMID: 3900181 [PubMed - indexed for MEDLINE]''