Source of thyroid hormones in peripheral tissues: All T4 in peripheral tissues originates from thyroid secretion, while about 20% of T3 comes from thyroid secretion and 80% from the conversion of T4 into T3 in peripheral tissues (by deiodinase deiodination).
Hormones in the blood: most of them exist in combination with proteins, and the amount of free hormones is very small: free T4 accounts for about 0.02% of total T4 and free T3 accounts for about 0.3% of total T3. Only free hormones are biologically active, while bound hormones play a transport role, and the dynamic balance between bound and free is maintained.
Change of thyroid hormones in hyperthyroidism (before treatment)
TSH: Generally, it is the first to decrease, mostly before the other four are elevated to undetectable.
TT3, FT3: next to be elevated, especially in those with poor iodine supply and relapse after surgery or radiotherapy.
TT4, FT4: generally elevated last. In iodine-induced (or high iodine reserve) hyperthyroidism, TT4 and FT4 are predominantly elevated, and precede TT3 and FT3.
Hyperthyroidism hormonal changes (after drug treatment)
General pattern: TT4 and FT4 fall first, then TT3 and FT3 decrease, TSH recovers last, often the other four items have returned to normal (even TT4 and FT4 are already low), TSH decrease can still last for quite a long time.
In combination with euthyroxine (L-thyroxine), the decrease in TT4 and FT4 may be synchronized with or even lag behind TT3 and FT3.
In combination with thyroid tablets, TT4 and FT4 will take the lead in decreasing during hyperthyroidism treatment and can often be lower than normal when hyperthyroidism is under control, but TT3 and FT3 are normal at this time.
Thyroid hormone changes in primary hypothyroidism
Before treatment: Generally, serum TSH is the first to rise, mostly significantly higher than normal before the other four indicators are lowered (subclinical hypothyroidism), then TT4 and FT4 are lowered, and finally TT3 and FT3 are lowered.
For those treated with thyroid tablets: TT3 and FT3 rise first, followed by a decrease in TSH to normal, and finally TT4 and FT4 rise to normal. In some patients, the symptoms of hypothyroidism have disappeared, TT3, FT3 and TSH are completely normal, and TT4 and FT4 can still be lower than normal. At this time the replacement dose is sufficient.
In patients treated with eugenol (L-thyroxine), TT4 and FT4 may rise simultaneously with TT3 and FT3, followed by a decrease in TSH until they return to normal. In some patients, when the clinical symptoms of hypothyroidism have disappeared and TT3, FT3 and TSH have reached normal levels, TT4 and FT4 may be mildly higher than normal, which does not reflect an excessive replacement dose.
Thyroid hormone changes in thyroiditis
Diseases: Subacute thyroiditis, painless thyroiditis, acute attacks of Hashimoto (chronic lymphocytic) thyroiditis, acute suppurative thyroiditis, etc.
Mechanism: The thyroid cells are destroyed by inflammation, and the thyroid hormone and thyroglobulin stored in the cells and follicles are released into the blood together, resulting in a rapid increase in the level of hormones in the blood circulation.
Features: The onset of the disease is often rapid, the duration of the disease is short, and the symptoms of hyperthyroidism are mild, but the elevation of serum TT4, FT4 and TT3 and FT3 is often more prominent than the clinical symptoms, with TT4 and TT3 elevated more significantly and TSH reduced. It is usually not necessary to use special anti-thyroid treatment, and the thyroid function will return to normal as thyroiditis improves.
Hormonal changes in pituitary secondary hypothyroidism
Before treatment: generally lower serum TT4 and FT4 are predominant and appear first, and later TT3 and FT3 decline. Serum TSH remains in the normal range in most patients, or may even be slightly high, and in a few patients TSH may be reduced.
L-T4 replacement therapy: With the increase of replacement amount, TT4, FT4 and TT3, FT3 will rise simultaneously, with TT4, FT4 rising more obviously, when the symptoms of hypothyroidism disappear, TT3, FT3 reach normal level, TT4, FT4 can be slightly higher than normal, TSH remains normal or lower than normal.
Thyroid tablet replacement therapy: With the increase of replacement dose, TT3 and FT3 will gradually rise, TT4 and FT4 will slowly rise, and when clinical hypothyroidism symptoms disappear, TT3 and FT3 will reach normal level or even slightly higher, while TT4 and FT4 can still be lower than normal, and TSH will remain normal or lower than normal.