How Your Thyroid Works - Controlling hormones essential to your metabolism
Almost all of the T3 (and T4) found in the blood is bound to protein. . American Association of Clinical Endocrinologists: About Your Thyroid. T4 and T3 are hormones produced by the thyroid gland. This article explains the The relationship between T4 and TSH levels. Image source. The thyroid really produces one active hormone: triiodothyronine, or T3 T4 is a pro-hormone. It has very little biological activity. Cells (mostly in the liver and.
These cells combine iodine and the amino acid tyrosine to make T3 and T4.
Table 6, Relationship between serum T4 and T3 in various disorders* - Endotext - NCBI Bookshelf
T3 and T4 are then released into the blood stream and are transported throughout the body where they control metabolism conversion of oxygen and calories to energy. Every cell in the body depends upon thyroid hormones for regulation of their metabolism. The thyroid gland is under the control of the pituitary gland, a small gland the size of a peanut at the base of the brain shown here in orange.
Under the influence of TSH, the thyroid will manufacture and secrete T3 and T4 thereby raising their blood levels. The pituitary senses this and responds by decreasing its TSH production. One can imagine the thyroid gland as a furnace and the pituitary gland as the thermostat.
Thyroid hormones are like heat. When the heat gets back to the thermostat, it turns the thermostat off. As the room cools the thyroid hormone levels dropthe thermostat turns back on TSH increases and the furnace produces more heat thyroid hormones.
How Your Thyroid Works
The pituitary gland itself is regulated by another gland, known as the hypothalamus shown in the picture above in light blue. However, in hyper- and hypo-thyroid states, a higher fraction of total plasma T3 is produced by the thyroid gland2.
T3 represents the active form of thyroid hormones. It increases cardiac output, cardiac contractility and heart rate and decreases systemic vascular resistance3. The remainder, with T4 levels towards the lower limit of normal, are subject to a progressive increase in the T3 fraction and may depend on it to remain euthyroid.
Thyroxine, in the form of Levothyroxine sodium, is the most widely prescribed treatment for hypothyroidism.
Combinations of thyroxine and triiodothyronine are available as synthetic preparations, but are not usually recommended. The main advantage of treatment with thyroxine is that the serum concentration of triiodothyronine, formed in extrathyroidal tissues from the ingested thyroxine, is controlled physiologically, which may be of benefit during illness or fasting when extrathyroidal production of triiodothyronine is usually decreased4.
Recent reports in hypothyroid patients receiving T4 replacement therapy have demonstrated T3 levels lower than those of euthyroid subjects, even when the T4 levels are in the high-normal range and TSH is suppressed5. We excluded patients with chronic or acute diseases, patients with iodopenic or nodular goiter, those on a weight-reducing diet, pregnant women, and patients taking drugs influencing thyroid function or serum hormone levels.
The age and the body mass index BMI of included patients, according to gender and thyroid status, are depicted in Table 1. As shown in this table, subjects from a wide range of age and body weight groups were used. The normal ranges for our laboratory are: Antithyroid antibodies were determined by two-site chemiluminescent immunoassay. Statistical analysis was performed with the statistical program SPSS v 8.
T4 levels were higher in subjects on Thyroxine replacement 9. T3 serum levels Table 3 were increased in hyperthyroid subjects and in those with subacute thyroiditis and decreased in hypothyroidism. There was no difference in T3 levels between euthyroids TSH levels Table 3 were increased in hypothyroid subjects and suppressed in hyperthyroid and those with DQ.
Thyroid serum hormone levels in subjects post thyroidectomy for carcinoma and with suppressed TSH were: More detailed characteristics of this subgroup are depicted in Table 8.
The nutritional status can influence the ratio if there is inadequate iodine intake or if the person is on a weight-reducing diet9. In the acute phase of DQ, the T4 and T3 levels are usually increased and the TSH levels are suppressed, but the differences from those of hyperthyroid subjects are not clear. In this study, our effort was to establish whether or not normal TSH levels in hypothyroid patients on replacement therapy with thyroxine alone reflects euthyroidism in all cases.
There is almost universal agreement on the use of L-thyroxine alone as replacement therapy for hypothyroidism.
Because of reports demonstrating that a low TSH concentration is a risk factor for the development of atrial fibrillation in the elderly10and a risk factor for osteoporosis and changes in the function of other target organs, such as the liver, heart and kidney11, the American Thyroid Association was prompted to state that "the goal of therapy with L-thyroxine is to restore patients to the euthyroid state and to normalise serum thyroxine and TSH concentrations"12, not mentioning at this time T3 levels.
In our study, the hypothyroid group on Thyroxine replacement had identical mean T3 concentrations with those of the euthyroid group