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Low-T3 syndrome and high-T3 syndrome: Consequences of step-up hypodeiodination, e.g. in critical illness as an example for type 1 allostasis, [20] or hyperdeiodination, as in type 2 allostasis, including posttraumatic stress disorder. [12] Resistance to thyroid hormone: Feedback loop interrupted on the level of pituitary thyroid hormone receptors.
TSH levels are determined by a classic negative feedback system in which high levels of T3 and T4 suppress the production of TSH, and low levels of T3 and T4 increase the production of TSH. TSH levels are thus often used by doctors as a screening test, where the first approach is to determine whether TSH is elevated, suppressed, or normal. [25]
As with hyperthyroidism, TSH is suppressed. Both free and serum (or total) T3 and T4 are elevated. [11] An elevation in thyroid hormone levels is suggestive of thyroid storm when accompanied by signs of severe hyperthyroidism but is not diagnostic as it may also correlate with uncomplicated hyperthyroidism.
If the TSH level is normal or low and serum free T 4 levels are low, this is suggestive of central hypothyroidism (not enough TSH or TRH secretion by the pituitary gland or hypothalamus). There may be other features of hypopituitarism , such as menstrual cycle abnormalities and adrenal insufficiency .
Fasting in healthy, euthyroid people causes reduced T3 and elevated rT3, although TSH is usually unchanged. [3] [5] [10] Even moderate weight loss can lower T3. [2] This may be primarily via reduced levels of leptin (the satisfaction hormone). Low leptin levels can downregulate hypothalamic TRH neurons and cause a reduction in TSH.
Thyroid-stimulating hormone (TSH) released from the anterior pituitary (also known as the adenohypophysis) binds the TSH receptor (a G s protein-coupled receptor) on the basolateral membrane of the cell and stimulates the endocytosis of the colloid. The endocytosed vesicles fuse with the lysosomes of the follicular cell.