Alterations in thyroid function are reported in obesity. Thyroid hormones and thyroid-stimulating hormone (TSH) concentrations have been variously described as normal, elevated, or low in morbidly obese patients compared with normal weight controls. However, it is a common observation that a significant proportion of patients with morbid obesity display slightly increased serum levels of TSH, while even relatively mild elevations of serum TSH are associated with an increase in the occurrence of obesity. Of note, abnormalities in thyroid function and TSH mostly normalize after weight loss, suggesting that these biochemical alterations are reversible. Despite uncertainty regarding the underlying mechanisms, it has been suggested that neither autoimmunity nor iodine deficiency seems to play a critical role. Several alternative mechanisms leading to hyperthyrotropinemia have been hypothesized, which include impaired feedback due to decreased number of triiodothyronine (T3) receptors in the hypothalamus, and variations in peripheral deiodinase activity. Leptin, in addition to regulating body weight and satiation, has also been shown to mediate the production of pro-TRH in cultured fetal rat hypothalamic neurons. Partial regulation of TSH by leptin has been also reported in humans. In addition, peripheral thyroid hormone metabolism appears to be reflected by the ratio of T3 to reverse T3 (rT3) (T3/rT3-ratio). We have shown that the T3/rT3-ratio is significantly increased in insulin resistant patients compared to their insulin sensitive partners despite comparable TSH values. Given that obesity is strongly associated with insulin resistance, and thyroid hormones are known to modulate carbohydrate metabolism, e.g. by affecting cellular glucose uptake, possible changes in the T3/fT3 ratio following weight loss after bariatric surgery could be of interest.

Data from cross-sectional studies further indicate that the thyroid structure of obese patients can be also affected, independent of the existence of autoimmune thyroiditis as indicated by the presence of auto-antibodies such as TPO. Ultrasound (US) scans are able to accurately characterize the echographic structure of thyroid tissue, in addition to estimation of thyroid volume and identification of non-palpable thyroid nodules. The typical normal thyroid parenchyma has a distinct high echo density due to the follicle structure, which contrasts well with tissue of the collar muscles. The interface between thyroid cells and the colloid exhibits elevated acoustic impedance, causing high-frequency acoustic waves to be reflected back to the US probe. However, in autoimmune thyroid diseases both lymphocytic infiltration and disruption of normal tissue architecture cause a reduction in thyroid echogenicity, whereas other tissues close by such as muscle tissue appear to remain unaffected. Only few previous studies reported on the morphology of the thyroid gland in adults with morbid obesity. Given that thyroid function has been reported to return to normal after weight loss, research questions are also raised about the potential reversibility of thyroid structural abnormalities following substantial weight loss in previously morbidly obese patients.