What are metabolic changes occurring during pregnancy leading to hypothyroidism?

Increased iodine renal clearance, increased TBG serum, inner ring deiodination of T3 and T4 by placenta, Stimulation by hCG, this causes a downwards shift in the normal range of TSH. TSH assessment based on population trimesters (first: lower range decreased by 0.4 and upper by 0.5, then we have gradual return towards non-pregnant range in second and third trimesters)