Thyroid Flashcards
What stimulates and inhibits TSH?
- Stimulated by TRH
- Inhibited by somatostatin and dopamine
What is the active thyroid hormone?
T3 is active thyroid hormone
3-5x more potent than T4
What is secreted at a higher rate? T3 or T4?
T4 is secreted at higher rate (x20) than T3
How is the majority of T3 made?
T3 is made primarily from peripheral conversion of T4 in the liver and kidney
Where is the thyroid hormone receptor?
Nucleus
What are the two key thyroid hormone receptors?
Nuclear steroid receptor; two types: alpha (chromosome 17), beta (chromosome 3)
Describe the implications of an inactivating mutation of the thyroid hormone receptor?
Inactivating mutation causes thyroid resistance syndrome
What is Hashimoto’s thyroiditis?
Most common cause of hypothyroidism in iodine-sufficient areas of the world is chronic autoimmune thyroiditis, which is caused by cell- and antibody-mediated destruction of thyroid tissue
What are the laboratory findings of primary hypothyroidism?
high TSH, low free T4
What are the main causes of primary hypothyroidism?
- Hashimoto’s thyroiditis
- Iatrogenic disease (thyroidectomy, radiation, etc)
Describe thyroid physiology in the setting of anorexia
- Similar to sick euthyroid state
- TSH normal or slightly low
- Low T3, high reverse T3
- Normal or low T4, but normal free T4
Describe Subacute (viral/deQuervain) thyroiditis:
- Subacute thyroiditis (subacute granulomatous thyroiditis) is characterized by neck pain or discomfort, a tender diffuse goiter, and a predictable course of thyroid function evolution
- Hyperthyroidism followed by euthyroidism, hypothyroidism, and ultimately restoration of normal thyroid function
- Low iodine uptake; high T3, low TSH
Describe secondary hypothyroidism
- TSH deficiency
- Hypopituitarism (tumor, Sheehan, trauma)
- TSH-R mutation
Describe tertiary hypothyroidism
- TRH deficiency
- Damage to hypothalamus or portal blood flow
- TRH-R mutation
Describe Graves Disease
- Autoimmune disease consisting of hyperthyroidism, goiter, eye disease (orbitopathy), dermopathy (pretibial or localized myxedema)
- Hyperthyroidism is the most common feature, affecting nearly all patients, and is caused by thyroid-stimulating hormone (TSH, thyrotropin)-receptor antibodies (TRAb) that activate the receptor, thereby stimulating thyroid hormone synthesis and secretion as well as thyroid growth (causing a diffuse goiter)
- High iodine uptake; high T3, low TSH
How do you work up a thyroid nodule?
- Serum TSH (higher TSH, higher risk of malignancy)
- Thyroid ultrasound
- FNA (if TSH is high and nodules meets sonographic criteria for sampling)
What is thyroid storm?
Rare, life-threatening condition characterized by severe clinical manifestations of thyrotoxicosis
Describe the treatment for thyroid storm?
- Beta blocker to control the symptoms and signs induced by increased adrenergic tone
- A thionamide to block new hormone synthesis
- An iodine solution to block the release of thyroid hormone
- An iodinated radiocontrast agent (if available) to inhibit the peripheral conversion of T4 to T3
- Glucocorticoids to reduce T4-to-T3 conversion, promote vasomotor stability, possibly reduce the autoimmune process in Graves’ disease, and possibly treat an associated relative adrenal insufficiency
What happens to TSH in pregnancy?
TSH – decreases in first trimester (secondary to HCG), then normalizes
What happens to total T4 in pregnancy?
Total T4 – increases; increased overall requirement (increased metabolic rate and some transfer to fetus)
What happens to free T4 in pregnancy?
Free T4 – decreases; due to increased TBG
What happens to TBG in pregnancy?
TBG – increase; secondary to increased estrogen levels
When does fetal synthesis begin?
8-10 weeks
When is fetal thyroid development is complete?
12- 14 weeks
