.2.Regulation of Thyroid function (1)Thyroid-pituitary relationships: (2)Autoregulation of the thyroid gland: 。 The thyroid gland also regulates its uptake of iodide and thyroid hormone synthesis by intrathyroidal mechanisms that are independent of TSH.These mechanisms are primarily related to the level of iodine in the blood
• 2．Regulation of Thyroid function • (1)Thyroid-pituitary relationships: (2)Autoregulation of the thyroid gland: • The thyroid gland also regulates its uptake of iodide and thyroid hormone synthesis by intrathyroidal mechanisms that are independent of TSH. These mechanisms are primarily related to the level of iodine in the blood
Acute Circadian and psychosis pulsatile rtythms Severe Cold ”stress Hypothalamus Somato- statin Corticoids dopamine TSH G Thyroid Figure 38-3.The hypothalamic-pituitary-thyroid axis. Acute psychosis or prolonged exposure to cold may activate the axis.Hypothalamic TRH stimulates pituitary TSH release,while somatostatin and dopamine inhibit it TSH stimulates T,and T3 synthesis and release from the thyroid,and they in turn inhibit both TRH and TSH syn- thesis and release.Small amounts of iodide are neces- sary for hormone production,but large amounts inhibit T3 and Ta production and release.(Solid arrows,stimula- tory influence;dashed arrows,inhibitory influence.H, hypothalamus,HPanterior pituitary.)
3.Transport ,degradation and excretion of thyroid hormones The liver is the major site of nondeiodinative degradation of thyroid hormones
• 3. Transport ,degradation and excretion of thyroid hormones The liver is the major site of nondeiodinative degradation of thyroid hormones
Table 38-1.Summary of thyroid hormone kinetics. Variable Volume of distribution 10L 40L Extrathyroidal pool 800g 54g Daily production 75g 25g Fractional turnover per day 10% 60% Metabolic clearance per day 11L 24L Half-life (bioloi 7days 1day Serum levels Total 5-11 ug/dL 95-190ng/dL (64-132 nmol/L) (1.5-2.9 nmol/L Free 07-185na/dL 0.2-052ng/dl (9-24 pmol/L) (3-8 pmol/L) Amount bound 99.96% 99.6% Biologpotency 1 4 Oral absorption 80% 95%