Table 4.

Other factors that modulate the effectiveness of LT4 (mechanisms unknown). (↓ = decrease, ↑ = increase)

Factor	Category	Subcategory	Observations/putative mechanism	Suggested action	References
Female sex	Pathophysiological	physiological	↑ Changes in estrogen levels during events such as pregnancy and menopause might decrease the FT4 concentration and thus reduce the entry of thyroid hormones into cells	Monitor the LT4 maintenance dose in female patients as a function of life events	[89–91]
Kidney transplantation	Pathophysiological	pathological	Kidney transplantation was associated with a marked fall in the required dose of LT4. Patients who subsequently return to dialysis required an LT4 dose increase	Decrease the LT4 dose after kidney transplantation	[124]
Food antigens	Pathophysiological	foodstuffs	↑ in immune cross-reactivity between thyroid hormones/enzymes and raw or cooked food products, as a possible trigger for thyroid disorders	Consider repeating the thyroid hormone assays with kits/techniques	[86]
Zinc	Pathophysiological	foodstuffs	↑ in serum FT3 after supplementation with Zn (alone or combined with Se)	Consider zinc supplementation in overweight or obese patients with hypothyroidism	[135]

FT3 free liothyronine, FT4 free thyroxine, LT4 levothyroxine, T4 thyroxine