Thyroid tests and papillary-follicular thyroid cancer follow-up successfully take advantage of the knowledge of how the normal thyroid gland functions. The purpose of the gland and its various cells is to produce exactly the right amount of two thyroid hormones to maintain a balanced metabolism.
The balance of hormone levels circulating in the blood involves a feedback loop to two areas of the brain. The hypothalamus makes thyroid releasing hormone (TRH), which travels to TRH receptors in the pituitary gland. The hormone then stimulates the pituitary to release another hormone, the thyroid stimulating hormone (TSH), into the circulation. The thyroid gland itself has follicle cell receptors that allow the TSH to stimulate thyroid follicular cells to produce both thyroxine (T4) and triiodothyronine (T3). Thyroglobulin (Tg), produced only by the thyroid, is a precursor of both hormones T4 and T3.
The two thyroid hormones T4 and T3 then circulate in the blood and inhibit synthesis of the brain hormones, TRH and TSH, as well as the TRH receptor in the pituitary, as part of a negative feedback loop. As the level of one set of hormones rises, the other set decreases.
Thyroid Cancer Typical Treatment
After pathologic diagnosis of cancer, the first step in treatment is usually total thyroidectomy. Surgery results in removal of the tumor, thereby reducing the risk of recurrence and making it easier to treat a smaller number of any remnant cancer cells.
The second step takes advantage of thyroid gland function, and targets only thyroid cells that use iodine to manufacture thyroid hormones. Even after total thyroidectomy, thyroid cells could remain in the thyroid bed, in cervical lymph nodes, or be located in metastases.
Radioactive iodine (I-131) is administered, and taken up only by functioning thyroid cells, whether normal or neoplastic. The functioning cells are killed by the radiation in the iodine, but there is not enough radiation to kill other tissues of the body. Radioactive Iodine (I-131) ablation of thyroid cancer cells was the first great success in targeted cancer treatment.
Once there are presumably no more thyroid cells, future scans make detecting a recurrence of cancer possible at an early stage.
The third step of treatment is replacement of the now-missing essential hormones. Thereafter, good follow-up means testing to detect any recurrences.
Thyroxine (T4) is easily monitored and replaced by Synthroid or other formulations which act identically to natural thyroxine.
Thyroxine, natural or exogenous, does two things: first, it restores the patient to the euthyroid state; and second, Thyroxine suppresses the pituitary hormone, TSH, by being part of the normal feedback loop from thyroid to hypothalamus to pituitary. Low levels of TSH are advantageous because any remnant cancer cells will not be stimulated to grow. After years of controversy, most writers agree that replacement of triiodothyronine (T3) is not needed. Frequently, the brand name for exogenous T3 seen in attending physicians’ reports is Cytomel.
Good follow-up means looking for possible thyroid cancer recurrences, and one key to this is ultrasound scans of the neck. Ultrasound is useful for all pathologic types of recurrent cancer in the neck. Knowledge of cell function, however, can aid in detection of recurrences of papillary/follicular types of cancer in all areas of the body, including the neck. The fact that these thyroid cells take up iodine and/or produce hormones creates another way to look for recurrences.
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