Final answer:
The primary causes of hypothyroidism include decreased synthesis of thyroid hormone, often due to Hashimoto's thyroiditis or iodine deficiency, with T3 and T4 levels being affected.
Step-by-step explanation:
The primary causes of hypothyroidism are often related to issues with the thyroid gland itself. The correct answer to this question is 1) Decreased synthesis of thyroid hormone.
This condition can be caused by a variety of factors, including Hashimoto's thyroiditis, dietary iodine deficiency, and damage or removal of the thyroid gland.
Hashimoto's thyroiditis, an autoimmune disease where the immune system attacks the thyroid, is the most common cause, particularly in areas where iodine deficiency is not prevalent due to iodine being added to salt.
In areas with inadequate iodine intake, deficiency can result in hypothyroidism, as the thyroid cannot produce adequate amounts of the hormones T3 and T4 without iodine.
Contrastingly, hyperthyroidism is indicated by 2) Increased synthesis of thyroid hormone, typically due to conditions such as Graves' disease.
Both hypothyroidism and hyperthyroidism can result in goiter, which is a visible enlargement of the thyroid gland.
In hypothyroidism, the increase in thyroid-stimulating hormone (TSH) as a response to low hormone levels can stimulate growth, while in hyperthyroidism, direct stimulation by antibodies as seen in Graves' disease can do the same. Hormones produced by the thyroid gland include T3 and calcitonin.