Endocrine Function II: Thyroid, Sex, and Gastrointestinal Hormones; Regulation of Calcium and Phosphorus Levels Flashcards
List three thyroid binding proteins
- Throxine binding globulin (TBG)
- Transthyretin (Thyroxine Binding Prealbmin) (TBPA)
- Albumin
Which thyroid hormone secreted in the greatest quantity?
T4 is the main secretory prodct
Which thyroid hormone is the most potent, biologically active?
T3
Describe the structure and clinical significance of increased levels of reverse T3
Biologically inactive form favored under stress
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List eight physiological effects of thyroid hormones
- Increased oxygen consumption
- Increases basal metabolic rate
- Stimulates heart rate/contraction
- Stimulates protein synthesis
- Stimulates all aspects of lipid metabolism
- Affects all aspects of CHO metabolism (4 “Gs”)
- Increases demands for vitamins and coenzymes
- Provides negative feedback to TRH and TSH
What is the cause of hyperthyroidism in Graves’ Disease?
Autoimmune etiology
- AutoAbs bind to TSH receptors in thyroid
- Body thinks it’s getting TSH but is fooled by Abs
What is the cause of hyperthyroidism in toxic multinodular goiters?
- Discrete portions of thyroid are not under normal feedback control
- No exophthalmopathy but still producing thyroid hormone
What is the cause of hyperthyroidism in solitary toxic adenoma?
- Patients have thyroid nodules that avidly concentrate injected radioactive iodine
- Benign tumor doesn’t respond to normal feedback control
Nodules that concentrate injected radioactive iodine
“Hot nodules”
What causes hyperthyroidism in subacute thyroiditis?
- Inflammation of the thyroid gland
- Appears to be viral in origin
- Follicles become inflamed and disrupted
What causes hyperthyroidism in iatrogenic symptoms?
Adverse mental or physical conditions caused by a medical procedure/physician
What causes hyperthyroidism in factitious symptoms?
- Disorders that aren’t genuine or natural
- Physical or psychological symptoms are produced under the voluntary control of the patient
What causes hyperthyroidism in TSH-dependent hyperthyroidism?
Caused by excess placental hormones (hCG) and TSH-secreting pituitary tumors (tertiary in hypothalamus)
List the symptoms associated w/ hyperthyroidism
- Heat intolerance
- Flushing
- Perspiration
- Increased appetite
- Weight loss
- Tachycardia
- SOB
- Nervousness
- Sometimes exophthalmopathy
TSH-independent hyperthyroidism
- T3 levels
- T4 levels
- TSH levels
- T3 levels: ↑
- T4 levels: ↑
- TSH levels: ↓
TSH-dependent (2°) hyperthyroidism
- T3 levels
- T4 levels
- TSH levels
- T3 levels: ↑
- T4 levels: ↑
- TSH levels: ↑
TSH-dependent (3°) hyperthyroidism
- T3 levels
- T4 levels
- TSH levels
- T3 levels: ↑
- T4 levels: ↑
- TSH levels: ↑
Primary hyperthyroidism (TSH-independent) diseases
- Graves’ disease
- Toxic multinodular goiter
- Solitary toxic adenoma
- Subacute (“painless”) thyroiditis
- Iatrogenic
- Factitious
What is the treatment for hyperthyroidism?
Any anti-thyroid drugs, radio-iodine ablation, or thyroidectomy
List the symptoms of hypothyroidism
- Cold intolerance
- Dry skin
- Decreased appetite
- Muscle weakness
- Slow heart rate
- Low BP
- Weight gain
- Hoarseness due to thickened vocal cords
- More common in women
What causes hypothyroidism?
- Diseases (commonly autoimmune) or treatments that destroy thyroid tissue or interefere w/ thyroid hormone biosynthesis
- Less often caused by secondary (pituitary) or tertiary (hypothalamic) disorders
Primary hypothyroidism diseases
- Adult hypothyroidism
- Cretinism (pluts endemic goiter)
- Hashimoto’s thyroiditis
Secondary hypothyroidsm disorders
Pituitary or hypothalamus disease/removal/destruction
What are the five causes of adult hypothyroidism?
- Intrinsic disease of the thyroid
- Total thyroidectomy
- Complete blockage or thyroid function by irradiation or an antithyroid drug
- Various diseases
- Myxedema causes face puffiness, doughy skin, subcutaneous emea
What is the cause of cretinism (neonatal hypothyroidism)
- Failure to develop thyroid gland in utero
- If mother receives anti-thyroid drugs or radioactive iodine during pregnancy
- If maternal antithyroid Abs cross placenta (results in mental retardation)
What is the cause of hypothyroidism due to iodine deficiency (endemic goiter)
- W/ little or no iodine, less hormone is made to provide negative feedback to TSH, which is continually trying to stimulate hormone production
- This eventually causes hyperplasia and large goiters
What causes Hashimoto’s thyroiditis?
- Most common cause of hypothyroidsm
- Autoimmune, chronic inflammatory disease of thyroid in
T-hleper cells stimulate B-lymphs to produce antithyroid Abs - Defect in organification, causing lymphocytic infiltration of thyroid
What causes secondary/tertiary hypothyroidism?
Any factor that affects TSH or TRF production
- Infection
- Surgical removal of pituitary or hypothalamus
- Trauma
- Cancer
- Autoimmune destruction of pituitary and/or hypothalamus
Primary hypothyroidism
- T3 levels
- T4 levels
- TSH levels
- T3 levels: ↓
- T4 levels: ↓
- TSH levels: ↑
- TRF (TRH) levels: ?
Secondary hypothyroidism
- T3 levels
- T4 levels
- TSH levels
- TRF (TRH) levels
- T3 levels: ↓
- T4 levels: ↓
- TSH levels: ↓
- TRF (TRH) levels: ?
Tertiary hypothyroidism
- T3 levels
- T4 levels
- TSH levels
- TRF (TRH) levels
- T3 levels: ↓
- T4 levels: ↓
- TSH levels: ↓
- TRF (TRH) levels: ?
Define euthyroidism
- Condition in which the serum total thyroid hormone concentrations are abnormal, WITHOUT evidence of clinical thyroid disease
- Reverse T3 levels are elevated until recovery
What causes euthyroidism?
- Pathogenesis is unknown
- May include decreased peripheral conversion of T4 to T3 and decreased binding of thyroid hormones to TBG
- Proinflammatory cytokines may be responsible for some of these changes
Diagnosis of euthyroidism vs. hypothyroidism
- Slight TSH elevation occurs during euthyroid recovery
- Elevation of TSH (> 30 mU/mL) indices true hypothyroidism
Euthyroidism
- T3 levels
- T4 levels
- TSH levels
- Reverse T3
- T3 levels: ↓
- T4 levels: ↓
- TSH levels: normal
- Reverse T3: ↑ (slight ↑ during recovery)
What is the TRH (TRF) stimulation test used for?
- To distinguish b/w 2° and 3° hypothyroidism
- Synthetic TRH is administered and TSH response is monitored
- If no TSH increase → 2°
- If significant TSH increase → 3°
What do the total T4/T3 assays measure?
Measure both free and bound hormone
What is the clinical use of total T3 test?
Use for diagnosis and monitoring of hyperthyroid patients w/ suppressed TSH levels and normal FT4 levels
This assay is the most sensitive and specific measure of thyroid activity
TSH
What is the clinical use of the thyroglobulin test?
- Primarily to monitor patients w/ FSH cancer (which would be increased)
- Storage form of thyroid hormone precursors
What is the clinical use of the T3 Uptake test w/ thryoid hormone binding ratio (THBR)?
- Clinical use is to directly or indirectly assess available TBG binding sites, but is not used to diagnose hyperthyroidism or hypothyroidism
- May be ordered if the patient has an abnormal total T4 or total T3 levels w/ normal TSH
- The “discrepancy” may be due to ↑ or ↓ TBG levels → patient’s thyroid is functioning just fine
What is the clinical use of total T4?
This test by itself, does not provide enough clinical information and must be reported in conjunction with the other thyroid tests
THBR
This test provides an approximation of the free hormone concentration in the presence of abnormal TBG levels as TBG or free hormone levels are not easily measured
What factors increase TBG levels?
- Pregnancy
- Estrogen replacement therapy
- Oral contraceptives
- Porphyria
- Hydatidiforme mole
- Heroine or methadone abuse
- Clofibrate
What factors decrease TBG levels?
- Androgens
- Anabolic steroids
- Nephrotic syndrome
- Cirrhosis
- Corticosteroids
- Cushing’s syndrome
- Acromegaly
How does the THBR differentiate true hypothyroidism or hyperthyroidism and states of ↑ or ↓ TBG concentrations w/ normal thyroid function
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Primary hyperthyroidism (thyroid problem)
- Total T3
- Total T4
- FT3
- FT4
- TSH
- THBR
- Total T3: ↑
- Total T4: ↑
- FT3: ↑
- FT4: ↑
- TSH: ↓
- THBR: ↓
Primary hypothyroidism (thyroid problem)
- Total T3
- Total T4
- FT3
- FT4
- TSH
- THBR
- Total T3: ↓
- Total T4: ↓
- FT3: ↓
- FT4: ↓
- TSH: ↑
- THBR: ↑
Secondary hyperthyroidism (pituitary problem)
- Total T3
- Total T4
- FT3
- FT4
- TSH
- THBR
- Total T3: ↑
- Total T4: ↑
- FT3: ↑
- FT4: ↑
- TSH: ↑
- THBR: ↓
Secondary hypothyroidism (pituitary problem)
- Total T3
- Total T4
- FT3
- FT4
- TSH
- THBR
- Total T3: ↓
- Total T4: ↓
- FT3: ↓
- FT4: ↓
- TSH: ↓
- THBR: ↑
Tertiary hyperthyroidism (hypothalamus problem)
- Total T3
- Total T4
- FT3
- FT4
- TSH
- THBR
- Total T3: ↑
- Total T4: ↑
- FT3: ↑
- FT4: ↑
- TSH: ↑
- THBR: ↑
Tertiary hypothyrodism (hypothalamus problem)
- Total T3
- Total T4
- FT3
- FT4
- TSH
- THBR
- Total T3: ↓
- Total T4: ↓
- FT3: ↓
- FT4: ↓
- TSH: ↓
- THBR: ↓
What is the specific diagnostic usefulness of the antithyroidglobulin Ab (TgAb)?
- Hashimoto’s thyroiditis >85% reactivity
- Graves’ Disease >30% reactivity
- Tumor marker for recurrence of thyroid cancer
What is the specific diagnostic usefulness of the thyroid peroxidase autoAb (TPOAb)?
- Hashimoto’s thyroiditis 100% reactivity
- Graves’ Disease 70-80% reactivity
- Ab itself may be cytotoxic to the thyroid
What is the specific diagnostic usefulness of the TSH receptor autoAb (TRAb)?
- Igs that bind to thyroid cell membranes at or near the TSH-receptor site
- Either cause hyperactivity of the thyroid (Graves’) or an inability for TSH to stimulate the thyroid (Hashimoto’s)
- Used for differential diagnosis of hyperthyroidism
- Used to predict fetal and neonatal thyroid dysfunction due to transplacental passage of maternal TRAb
- Used to predict the course of Graves’ disease patients on antithyroid drug therapy
of carbons present in estrane
18 carbons
of carbons present in androstane
19 carbons