Thyroid Gland Hormones

Which test should be ran first when testing for hypothyroidism?

TSH because the level of TSH will indicate whether symptoms are due to thyroid gland or anterior pituitary gland.

Secondary Hyperthyroidism

TSH-dependent, i.e. due to pituitary gland problem. Lab results indicate elevated T3 and T4 and TSH.

Primary hyperthyroidism

TSH-independent, i.e. due to thyroid gland problem (cancer, hyperplasia goiter, inflammation due to infection). Lab results show elevated T3 and T4 and low-to-none TSH.

Hashimoto's Disease (Chronic Immune Thyroiditis)

Genetic autoimmune disorder that causes lymphocytes to infiltrate the thyroid gland and render it unproductive. Primary cause of hypothyroidism.

Grave's Disease

Genetic autoimmune disorder that results in hyperthyroidism. Antibodies bind to TSH receptors on thyroid gland and cause it to secrete thyroid hormone. Lab results show elevated T3 and T4, and normal or slightly decreased TSH

Thyrotoxicosis

Hyperthyroidism. Excess circulating T3 and T4. Causes can be primary (thyroid gland) or secondary (pituitary).

Thyroiditis

Inflammation of thyroid gland due to infection.

Regulation of T3 and T4

Negative feedback. Low levels of T3 and T4 cause pituitary glland to secrete more TSH, while too much T3 and T4 works to inhibit pituitary release of TSH, shutting down the synthesis of T3 and T4.

Thyroid Hormone Functions

Regulates carb, lipid, and protein metabolism within cells. Nervous system development and maintenance Growth and development Stimulates heart contractions

T3 and T4 transport

Bound T3 and T4 is bound to thyroxine-binding globulin (TGB) or TB albumin. Some hormone remains free

Primary Hypothyroidism

Caused by absence of or damage to thyroid gland. Lab results show decreased serum T3 and T4 and increased TSH

Secondary Hypothyroidism

Caused by faulty pituitary gland resulting in decreased production of TSH. Lab results show low TSH, T3, and T4.

Athyreosis

Complete absence of thyroid gland

Stimulation of T3 and T4

thyrotropin-releasing hormone (TRH) is released by hypothalamus, which causes pituitary to release thyroid-stimulating hormone (TSH, also called thyrotropin), which causes thyroid uptake of iodine, releasing T3 and T4.

Thyroid gland

Bi-lobed gland containing follicular cells that produce Thyroxine (T3) and triiodothyronine (T4), and parafollicular cells that produce calcitonin