Hyperthyroidism
Background on Thyroid Function
- The purpose of the thyroid gland is to produce thyroid hormone which is released into the blood and then circulates throughout the body.
- Thyroid hormone helps to control metabolism, effects growth and development, controls body temperature, and helps to regulate the cardiovascular system among many other important functions in the body.
- There are two forms of the hormone that are produced: T4 (thyroxine) and T3 (triiodothyronine), accounting for 80% and 20% of hormone in the blood respectively.
- The production of thyroid hormone is controlled by the brain (specifically the pituitary gland).
- The brain continuously measures thyroid hormone levels in the blood, and produces a hormone called TSH (thyroid stimulating hormone or thyrotropin) when more thyroid hormone is needed.
- Levels of TSH and T4 / T3 in the blood have an inverse relationship, or teeter totter effect:
- When T4 and T3 levels go down, TSH levels will go up to compensate, and vice versa.
- This is a tightly regulated feedback loop (or hormone axis in the body).
Hyperthyroidism
- Diagnosis:
- Overt hyperthyroidism (aka thyrotoxicosis) is defined by elevated free T4 and total T3, with low TSH levels in the blood.
- Subclinical hyperthyroidism is defined by high-normal free T4 and total T3, with a low TSH.
- TSH level measurements can be affected by biotin supplements (commonly present in hair and nail supplements), and therefore these supplements should be stopped 1 week prior to blood draw.
- Symptoms include:
- Unexplained weight loss, rapid/irregular heartbeat, tremor, sweating, irritability, heat intolerance, insomnia, diarrhea, nervousness, anxiety.
- Goiter formation causing compressive symptoms (constant pressure in the neck, choking sensation, difficulty swallowing or breathing, stridor).
- Long-term consequence of untreated Hyperthyroidism:
- Weight loss, osteoporosis (progressive bone thinning), atrial fibrillation (uncontrolled fast heart arrythmia), blood clots, muscle weakness, tremor, neuropsychiatric symptoms (anxiety, depression), rare cardiovascular collapse and death.
Graves’ Disease.
- Syndrome of hyperthyroidism, goiter (enlarged thyroid gland), ophthalmopathy or exophthalmos (bulging eyes or Thyroid Eye Disease), and dermopathy (pretibial myxedema – swelling and discoloration of the lower leg skin).
- Considered an auto-immune disease where the body’s immune system attacks the thyroid, mistaking it for something foreign to the body.
- Antibodies are made specifically to attack the thyrotropin (TSH) receptor, referred to as Thyrotropin Receptor Antibodies (TRAb).
- Antibodies attacking this receptor stimulate thyroid hormone production and release into the blood independent of TSH levels made by the pituitary gland.
- Disrupts the normal tightly regulated feedback loop.
- Can also cause the thyroid gland to grow and form a goiter.
- Most common cause of hyperthyroidism in the U.S.
- More common in ages < 45.
- Can resolve in 30% of patients spontaneously or with medical treatment.
- Diagnosis:
- Diffuse intense uptake on thyroid uptake scan.
- Elevated Thyrotropin Receptor Antibody (TRAb) and Thyroid Stimulating Immunoglobulin (TSI).
- Exam with diffuse bilateral goiter formation.
- Exam with bulging eyes (orbitopathy).
Toxic thyroid nodule vs Toxic multinodular goiter.
- Caused by a single nodule or multiple nodules autonomously producing thyroid hormone independent of Pituitary Gland feedback mechanism and TSH levels.
- Thought to be caused by somatic (non-inherited) activating mutations of DNA genes regulating thyroid growth and hormone production.
- More common in ages > 45.
- More common in regions of iodine deficiency.
- Usually progressively worsens overtime and less likely to resolve spontaneously or with medical treatment.
- Diagnosis:
- Focal uptake in one or more discrete regions on thyroid uptake scan.
- Focal uptake corresponds to a discrete nodule on ultrasound.
Other rare causes for Hyperthyroidism.
- Familial non-autoimmune hyperthyroidism (genetically inherited).
- Amiodarone (prescription medicine used to treat heart arrythmias) induced thyrotoxicosis (5-10% of amiodarone patients).
Causes for temporary hyperthyroidism:
- Subacute thyroiditis.
- Inflammation (thought to be due to a virus) which causes release of stored thyroid hormone.
- Typically followed by a period of hypothyroid symptoms until thyroid hormone stores can be replenished.
- Can present with or without localized pain in the thyroid and/or fever.
- Thyroid Antibody levels and Thyroid Uptake Scan will both be normal.
- Post-partum thyroiditis.
- Brief episode of thyroid inflammation and release of thyroid hormone following delivery.
- Also typically followed by a period of hypothyroid symptoms until thyroid hormone stores can be replenished.
- Thyroid Antibody levels and Thyroid Uptake Scan will both be normal.
Labs
- In addition to TSH, free T4, and total T3:
- Thyrotropin Receptor Antibody (TRAb) and Thyroid Stimulating Immunoglobulin (TSI) can be measured in the blood.
- Should be elevated in Graves’ Disease but not in a toxic nodule or toxic multinodular goiter.
Imaging
- Radioiodine scan (‘thyroid uptake scan’).
- A low dose radioactive iodine (typically I-123) material is ingested via pill or fluid.
- This material is absorbed by overactive thyroid cells and is visible on special cameras taking pictures of the thyroid.
- Different patterns of iodine uptake will help differentiate between the possible causes of hyperthyroidism.
- A single toxic (or ‘hot’) nodule with be visible as focal uptake in one discrete area of the thyroid.
- It is very rare for hot thyroid nodules to be cancerous.
- Hot nodules therefore do not need to be biopsied (aka FNA) unless there are high risk features present on ultrasound.
- Multiple toxic (or ‘hot) nodules will be visible as focal uptake in multiple discrete areas of the thyroid.
- Presence of a hot nodule(s) should be confirmed with a discrete visible nodule on ultrasound in the same location.
- Diffuse uptake in both lobes of the thyroid is diagnostic of Graves’ disease.
- There should be no uptake if hyperthyroidism is temporary in nature:
- Subacute, post-partum thyroiditis, or amiodarone induced.
- Should be avoided in pregnant and breastfeeding patients.
- Can result in cretenism of the baby (complete loss of thyroid function).
Image Source: Medizzy
Ultrasound
- Should be obtained if:
- Toxic nodule(s) discovered on Thyroid Uptake Scan to correlate location for possible surgery.
- Graves’ Disease is diagnosed and surgery is being considered as a treatment option.
- Helps in surgical planning and anticipating the size of incision needed.
- Goiter (enlarged thyroid) is felt on exam.
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