When levels of T3 and T4 are low, the hypothalamus responds by releasing thyrotropin-releasing hormone (TRH) which then stimulates the pituitary gland to release the thyroid stimulating hormone (TSH).
Cold temperatures also have the ability to increase TRH levels. This is thought to be an intrinsic mechanism that helps keep us warm in cold weather.
Elevated levels of cortisol, as seen during stress and in conditions such as Cushing’s syndrome, lowers TRH, TSH and thyroid hormone levels.
The thyroid gland needs iodine and the amino acid L-tyrosine to make T4 and T3. A diet deficient in iodine can limit how much T4 the thyroid gland can produce and lead to hypothyroidism.
Various factors including nutrient deficiencies, drugs, and chemical toxicity may interfere with the conversion of T4 to T3.
Oral contraceptives, pregnancy, and conventional female hormone replacement therapy may increase thyroid carrier protein levels and, thereby, lower the amount of free thyroid hormone available. Very small changes in the amounts of carrier proteins will affect the percentage of unbound hormones.