Your TSH levels and T4 (Synthroid) levels may be stable, but you could be experiencing lower T4 to T3 conversion. T3 is the active thyroid hormone, so you could be essentially going more functionally hypothyroid without impacting your TSH level. TSH levels are governed by T4
"Moderate weight loss can decrease the amount of T3 in the body. This is because the body decreases its peripheral conversion of T4 to T3 when it's in caloric restriction mode."
Background: Thyroid hormones are important determinants of energy expenditure, and in rodents, adipose tissue affects thyroid hormone homeostasis via leptin signaling. The relationship between thyroid hormones and nutritional status in humans has ...
pmc.ncbi.nlm.nih.gov