Thyroid Research Thyroid Research Archive Obesity

Secretion of thyrotropin and thyroid hormone is slightly increased in morbidly obese subjects

(July 2006)

The background of the study. There is a positive association between thyrotropin (TSH) secretion within the normal range and body-mass index, but the relationships between TSH and thyroid hormone secretion are unclear. In this study, thyroid function was assessed and the relationships between thyroid function and insulin sensitivity were evaluated in subjects with morbid obesity.

How the study was done. The study subjects were 78 subjects with morbid obesity (body-mass index >40 kg/m2) who had normal serum TSH, total and free thyroxine (T4), total and free triiodothyronine (T3), and antithyroid peroxidase antibody concentrations. They and 77 age- and sex-matched normal-weight subjects had measurements of height, weight, waist circumference, and fat and lean body mass. An oral glucose-tolerance test was performed in 49 of the morbidly obese subjects, and their sensitivity to insulin was measured.

The results of the study. The 78 subjects with morbid obesity and normal thyroid function had higher serum TSH, total T4, total T3, and free T3, but not free T4, concentrations, as compared with the normal subjects (Table). There was no correlation between the subjects’ serum TSH concentrations and body-mass index, fat mass, lean body mass, or waist circumference.

Table. Mean Serum TSH and Thyroid Hormone Concentrations in Morbidly Obese and Normal-Weight Subjects.
Serum	Morbid Obesity (n = 78)	Normal-Weight (n = 77)	P Value
TSH (mU/L)	2.0	1.7	<0.05
Total T4 (µg/dl)	8.7	7.6	<0.01
Free T4 (ng/dl)	1.2	1.3	>0.05
Total T3 (ng/dl)	154	110	<0.01
Free T3 (ng/dl)	0.37	0.33	<0.05

Among the 49 morbidly obese subjects who had glucose-tolerance tests, 71 percent had normal glucose tolerance and 29 percent had impaired glucose tolerance. In the subjects with normal glucose tolerance, serum TSH concentrations were positively correlated with fasting serum insulin concentrations and with insulin resistance.

The conclusions of the study. Serum TSH and thyroid secretion concentrations are slightly higher in morbidly obese subjects than in normal-weight subjects, suggesting resistance of TSH secretion to the inhibitory action of T4 and T3.

The original article. Michalaki MA, Vagenakis AG, Leonardou AS, Argentou MN, Habeos IG, Makri MG, Psyrogiannis AI, Kalfarentzos FE, Kyriazopoulou VE. Thyroid function in humans with morbid obesity. Thyroid 2006;16:73-8.