Thyroid Hormone Features

The correct answer is D

The thyroid gland is made up of cells arranged into follicles. Each of these cells takes up inorganic iodine through a Sodium/Iodine symporter. This process in stimulated by TSH. Once the iodine is within the cell, thyroid peroxidase, an apical membrane protein, catalyzes a reaction to organify the iodine and move it into the follicular space (colloid).

While in this space, thyroid peroxidase (TPO) also catalyzes a reaction which links two of these molecules together, thus forming T3 and T4. These two steps which utilize TPO are important, because they are the site of function for the anti-thyroid medications PTU and methimazole, both of which inactivate TPO.

After hormone production, it is kept in the colloid until a stimulus (TSH) induces its release. The hormone is then endocytosed, the excess protein is removed, and then the hormone is secreted into the bloodstream. This release step is also affected by the medication lithium, and when taken chronically, can lead to goiter due to inability to release stored hormone.

Thyroid hormones are controlled through a feedback inhibition model, where the final product, T3, inhibits the release of thyrotropic factors. The initiating factor in the cascade is TRH, which is produced in the hypothalamus. This then stimulates the release of TSH. However, the release of TSH is influenced more by the circulating T3 levels than the TRH. Once TSH is produced, it then acts at on the thyrocytes directly to stimulate growth, iodine uptake, and colloid endocytosis. Once the colloid is endocytosed and released, it circulates as T3 and T4, with 98% of the hormone in circulation being T4.

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