Hypothyroidism Overview

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Questions and Answers

Which condition is NOT typically associated with Addison's disease?

Rheumatoid arthritis

Diabetes mellitus type 1

Celiac disease or gluten sensitivity

Cirrhosis (correct)

Which of the following factors can cause high TSH levels in lab tests?

Chronic liver disease

Pituitary TSH hypersecretion (correct)

Hyperthyroid treatment

Acute pancreatitis

What effect does biotin have on thyroid lab testing?

It causes falsely low levels of fT4 and fT3.

It increases the levels of TSH in the blood.

It does not affect thyroid function but interferes with lab results. (correct)

It enhances the accuracy of TSH measurements.

Which drug is noted for potentially causing low levels of T4 and T3?

Phenytoin Signup and view all the answers

What is a common cause of lab error in testing thyroid hormone levels?

Strenuous exercise Signup and view all the answers

Which of the following autoantibodies may cause interference in thyroid function tests?

Antithyrotropin antibodies Signup and view all the answers

What should patients do to avoid biotin interference in lab tests?

Discontinue biotin at least 48 hours prior to testing. Signup and view all the answers

Which condition is associated with the potential for low T4 and T3 as a result of severe illness?

Cirrhosis Signup and view all the answers

What is the recommended treatment for primary hypothyroidism?

Levothyroxine (T4 replacement therapy) Signup and view all the answers

What characterizes subclinical hypothyroidism requiring treatment?

TSH > 10 mIU/L with cardiovascular risk Signup and view all the answers

During pregnancy, what is a likely change in thyroid hormone requirements?

Increase of 20% to 40% as early as 4 weeks Signup and view all the answers

What is the typical TSH reference range during the second trimester of pregnancy?

0.1 - 4.0 mIU/L Signup and view all the answers

Which factor does NOT contribute to increased thyroid hormone requirements in pregnancy?

Decreased metabolism of maternal thyroxine Signup and view all the answers

What characterizes central hypothyroidism?

Insufficient stimulation of the thyroid gland by TSH. Signup and view all the answers

Which condition is considered subclinical hypothyroidism?

Elevated TSH with fT4 within the normal range. Signup and view all the answers

What is the recommended management for subclinical hypothyroidism when TSH exceeds 10 mIU/L?

Treatment with T4 is recommended. Signup and view all the answers

What is the most common diagnosis method for central hypothyroidism?

Serum TSH and fT4 levels. Signup and view all the answers

Which population is at the highest risk for developing subclinical hypothyroidism?

Females above 65 years old. Signup and view all the answers

What is a symptom of central hypothyroidism in adults?

Hypothyroid symptoms, often milder. Signup and view all the answers

What is the incidence rate of overt hypothyroidism during pregnancy?

0.3–0.5% Signup and view all the answers

Which of the following is NOT a characteristic of primary hypothyroidism?

Normal TSH with elevated fT4. Signup and view all the answers

Which of the following conditions can lead to central hypothyroidism?

Post-irradiation of the pituitary. Signup and view all the answers

Which complication associated with hypothyroidism in pregnancy has the highest risk increase with elevated antibodies?

Recurrent loss Signup and view all the answers

Which of the following effects does levothyroxine (LT4) NOT have on pregnancy outcomes?

Increases placental abruption risk Signup and view all the answers

What is the recommended increase in thyroxine during pregnancy for women with known hypothyroidism?

30% Signup and view all the answers

What percentage of pregnancies may exhibit subclinical hypothyroidism?

2–3% Signup and view all the answers

What is the most common cause of primary hypothyroidism in North America?

Chronic autoimmune thyroiditis Signup and view all the answers

Which type of hypothyroidism is characterized by inadequate thyroid stimulating hormone production?

Secondary hypothyroidism Signup and view all the answers

Which of the following symptoms is NOT typically associated with low thyroid hormone levels?

Hypertension Signup and view all the answers

What type of hypothyroidism is directly caused by medical examination or treatment?

Iatrogenic hypothyroidism Signup and view all the answers

Which of the following is a potential neurological symptom of hypothyroidism?

Myxedema coma Signup and view all the answers

Iodine deficiency is a common cause of primary hypothyroidism mainly in which regions?

Geographically iodine-deficient areas Signup and view all the answers

Which medication is known to potentially cause iatrogenic hypothyroidism?

Amiodarone Signup and view all the answers

Which of the following conditions can lead to central (tertiary) hypothyroidism?

Brain tumors compressing the hypothalamus Signup and view all the answers

What could contribute to the occurrence of transient hypothyroidism?

Subacute thyroiditis Signup and view all the answers

Which of the following findings is associated with hypothyroidism in the hematologic system?

Increased bleeding risk Signup and view all the answers

Study Notes

Hypothyroidism

A hypometabolic state where the thyroid gland fails to produce adequate thyroid hormones (T4 & T3) or the hypothalamus and pituitary gland fail to stimulate sufficient hormone production.

Types of Hypothyroidism

Primary: 95% of cases, thyroid gland dysfunction.
Secondary (Central): Inadequate TSH production.
Tertiary: Inadequate TRH production.
Iatrogenic: Thyroid dysfunction caused by medical examination or treatment.
Resistance to thyroid hormone: Rare.

Etiology of Primary Hypothyroidism

Iodine Deficiency: Common in regions lacking adequate iodine.
Autoimmunity: Most prevalent cause in North America.
- Hashimoto thyroiditis: Most common autoimmune thyroiditis.
- De Quervain thyroiditis: Rare.
Transient: Postpartum Thyroiditis, pregnancy, silent thyroiditis, subacute thyroiditis.
Congenital Abnormalities: Aplasia/agenesis of the thyroid, dyshormonogenesis.
Infiltrative Thyroid Diseases: Rare, including amyloid goiter, black thyroid, cystinosis, diffuse lipomatosis, hereditary hemochromatosis, Langerhans cell histiocytosis, Riedel's thyroiditis, sarcoidosis, and scleroderma.

Etiology of Central (Secondary + Tertiary) Hypothyroidism

Pituitary Tumors: Pituitary adenomas are most common.
Sheehan Syndrome: Rare condition causing pituitary gland damage after severe blood loss during childbirth.
Lymphocytic Hypophysitis: Rare, autoimmune condition affecting the pituitary gland.
Brain Tumors: Can compress the hypothalamus.
TRH Resistance/Deficiency: Issues with the production or response to TRH.
Radiation Therapy to the Brain: Can affect the hypothalamus and pituitary gland.

Iatrogenic Hypothyroidism

Medications:
- Amiodarone
- Antibiotics: Rifampin, Ethionamide
- Anticonvulsants: Phenytoin, Carbamazepine
- Anti-neoplastics: Tyrosine kinase inhibitors (Sunitinib, Imatinib), Bexarotene, Interleukin-2, Anti-CTLA-4 and Anti-PD-L1/PD-1
- Interferon-α
- Lithium
- Perchlorate
- Phenobarbital
- Stavudine
- Thalidomide
- Dopamine
- Opioids
- Prednisone
Procedures: Radiotherapy to the head or neck area, thyroid radioactive iodine therapy, thyroid surgery.

Impact of Low Thyroid Hormones on Body Functions

Skin: Reduced sweating, skin discoloration, coarse hair (or loss), brittle nails, non-pitting edema, periorbital edema.
Hematologic: Hypocoagulability (bleeding risk), pernicious anemia.
Cardiovascular: Bradycardia, pericardial effusion, diastolic hypertension.
Respiratory: Shortness of breath on exertion, rhinitis, decreased exercise capacity.
Gastrointestinal: Constipation, decreased taste, nonalcoholic fatty liver disease.
Reproductive: Menstrual irregularities, decreased libido, infertility, miscarriage, erectile dysfunction, delayed ejaculation, reduced sperm morphology.
Neurologic: Hashimoto encephalopathy, myxedema coma.
Muscular: Weakness, cramps, myalgias (high serum creatine kinase).
Mental: Depression, anxiety, poor concentration, decreased short-term memory.
Metabolic: Hyponatremia, hyperlipidemia, hypercholesterolemia, hyperhomocysteinemia, hyperuricemia, reduced drug clearance.

Central Hypothyroidism

Hypothyroidism due to insufficient TSH stimulation of a normal thyroid gland.
Occurs in 1 in 80,000-120,000 individuals, less than 1% of hypothyroid cases.
Can be secondary (pituitary) or tertiary (hypothalamus) in origin.
Children: Craniopharyngiomas, history of cranial irradiation.
Adults: Pituitary macroadenomas, pituitary surgeries, post-irradiation.
Transient: Sick euthyroid syndrome, over-replacement of T4 (primary hypothyroidism).
Hypothyroid symptoms (often milder).
Diagnosis: Serum TSH and fT4 (low fT4, relatively low TSH).
Management: Referral for a TRH Stimulation Test (Health Canada Special Access).
Prognosis: Similar to primary hypothyroidism.

Subclinical Hypothyroidism

Endocrine disorder with elevated TSH but normal fT4.
Affects 5-8% of females, 3% of males in the US, and 3-15% of the general population.
Increased risk in females, individuals over 65 years, and those with autoimmune thyroiditis.
- Often asymptomatic but can present with hypothyroid symptoms.
Diagnosis: Serum TSH and fT4 +/- symptoms +/- TPO antibodies (elevated TSH > 4.0 mIU/L, fT4 within range).
Management: Treatment recommended if TSH > 10 mIU/L, TPO antibodies present, patient is symptomatic or has cardiovascular risk factors.

Management of Hypothyroidism

Primary Hypothyroidism: Levothyroxine (T4 replacement therapy).
Subclinical Hypothyroidism: Consider levothyroxine if TSH > 10 mIU/L, positive thyroid antibodies, cardiovascular risk or treatment-resistant depression.
Secondary Hypothyroidism: Brain MRI.

Thyroid Function in Pregnancy

Thyroid hormone requirements increase by 20-40% as early as 4 weeks of pregnancy.
- Increased estrogen-mediated thyroid-binding globulin, increased volume of distribution, placental metabolism, and transport of maternal thyroxine.
- Thyroid gland size increase in about 15% of pregnant women.

Hypothyroidism in Pregnancy

Affects 0.3-0.5% of pregnancies (overt hypothyroidism), and 2-3% (subclinical).
Predominantly chronic autoimmune hypothyroidism.

Miscarriage: Increased risk with elevated thyroid antibodies.
Recurrent Loss: Increased risk with elevated antibodies in euthyroid individuals.
Preterm Delivery: Increased risk with elevated antibodies (medicated).
Placental Abruption: Increased risk, especially in the first and second trimester.
Pre-eclampsia: Increased risk with primary hypothyroidism (medicated).
Postpartum Thyroiditis: Increased risk.
Lower Offspring IQ: Possible negative effect.

Thyroid Hormone Replacement Therapy (Levothyroxine, LT4)

Little to no effect on hypertensive disorders or placental abruption.
Reduces the risk of miscarriage and preterm birth.
Improves fetal intellectual development.

Natural Health Products

Biotin: Can falsely elevate fT4 and fT3 while lowering TSH.
St. Johns' Wort: Suggests potential association with transiently elevated TSH levels.

Important Considerations

Pregnancy: Increase thyroxine by 30% once pregnancy is confirmed unless preconception TSH was normal.
Natural Health Products: Be mindful of potential interactions with thyroid function tests, especially biotin.
Central Hypothyroidism: Need referral for TRH Stimulation Test.
Subclinical Hypothyroidism: Consider treatment for elevated TSH if necessary.
Management: Adjust levothyroxine dosage as needed to maintain TSH within the normal range.

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Description

Test your understanding of hypothyroidism, its types, and causes. This quiz covers primary, secondary, and tertiary hypothyroidism, as well as various etiological factors. Perfect for medical students or anyone interested in endocrine health.