High circulating thyrotropin levels in obese women are reduced after body weight loss induced by caloric restriction.

CONTEXT:

Previous clinical studies concerning the impact of body weight loss on single plasma TSH concentration measurements or the TSH response to TRH in obese humans have shown variable results.

OBJECTIVE:

The objective of this study was to investigate the effect of weight loss induced by caloric restriction on diurnal TSH concentrations and secretion in obese humans.

DESIGN:

This was a clinical, prospective, crossover study.

SETTING:

The study was conducted at the Clinical Research Center of Leiden University Medical Center.

PARTICIPANTS:

Eleven obese premenopausal women (body mass index, 33.3 +/- 0.7 kg/m2) were studied.

INTERVENTION:

The study intervention was weight loss (50% reduction overweight by caloric restriction).
MAIN OUTCOME MEASURE(S): Twenty-four-hour plasma TSH concentrations (10-min intervals) and the 24-h TSH secretion rate, calculated by a waveform-independent deconvolution technique (Pulse), were determined.

RESULTS:

The 24-h TSH secretion rate was significantly higher in obese women than in normal weight controls, and weight loss was accompanied by diminished TSH release (before weight loss, 43.4 +/- 6.4 mU/liter.24 h; after weight loss, 34.4 +/- 5.9 mU/liter.24 h; P = 0.02). Circulating free T3 levels decreased after weight loss from 4.3 +/- 0.19 to 3.8 +/- 0.14 pmol/liter (P = 0.04). Differences in 24-h TSH release correlated positively with the decline of circulating leptin (r2 = 0.62; P < 0.01).

CONCLUSIONS:

Elevated TSH secretion in obese women is significantly reduced by diet-induced weight loss. Among various physiological cues, leptin may be involved in this phenomenon. The decreases in TSH and free T3 may blunt energy expenditure in response to long-term calorie restriction, thereby frustrating weight loss attempts of obese individuals.