Hypothyroidism: Diagnosis and Physiological Effects

• Thyroid disorders involve conditions affecting the thyroid gland
• The thyroid gland, located in the neck, is responsible for producing hormones that regulate metabolism, energy levels, and overall body function

Prevalence & Impact of Thyroid Disease

• Over 12% of the U.S. population will develop a thyroid condition during their lifetime.
• 20 million Americans have some form of thyroid disease
• Women are five to eight times more likely than men to have thyroid problems.
• One in eight women will develop a thyroid disorder during their life.
• Thyroid disorder prevalence increases with age and is more common in adults over 65 years.

Physiologic Effects of Thyroid Hormones

• Promotes basal metabolic function, regulates O2 consumption, and heat production
• Affects cardiovascular muscle contraction
• Stimulates bone resorption and bone formation
• Allows normal glucose metabolism, absorption, and storage
• Functions in the synthesis and breakdown of lipids
• Affects the rates of metabolism of many hormones and drugs

Hypothalamic-Pituitary-Thyroid Axis

• Low serum levels of T3 and T4 trigger Thyrotropin Releasing Hormone (TRH) release from the hypothalamus.
• TRH causes Thyroid Stimulating Hormone (TSH) release from the pituitary.
• TSH triggers increased release of thyroid hormones until a normal serum level is reached

Hypothyroidism

• Hypothyroidism is a condition where thyroid hormone synthesis is insufficient for the body's needs.
• Primary Hypothyroidism occurs when diseases or treatments destroy thyroid tissue or interfere with thyroid hormone biosynthesis.
• Secondary Hypothyroidism results from disorders of the pituitary gland or hypothalamus.
• Subclinical hypothyroidism is an asymptomatic condition with elevated serum thyroid-stimulating hormone (TSH) but normal thyroxine (T4).

Common Causes of Hypothyroidism

• Chronic autoimmune (Hashimoto) thyroiditis
• Surgical removal of the thyroid
• Radiation treatment
• Medications like amiodarone, lithium, interferon alpha, and interleukin-2

Risk Factors for Elevated TSH

• Female sex
• Pituitary or hypothalamic disease
• Advancing age
• White race
• Type 1 diabetes
• Down syndrome
• Family history of thyroid disease
• Goiter
• Previous hyperthyroidism (ablation therapy that leads to iatrogenic thyroid dysfunction)
• External-beam radiation in the head and neck area

Classic Presentation of Hypothyroidism

• Cold intolerance
• Weight gain
• Menstrual abnormalities
• Constipation
• Forgetfulness
• Depression
• Fatigue

Physical Examination Findings in Hypothyroidism

• Delayed deep tendon reflexes
• Coarse, dry skin
• Brittle nails
• Hair loss
• Goiter
• Bradycardia
• Hoarseness

Differential Diagnoses for Hypothyroidism

• Depression
• Chronic Nephritis
• Chronic Fatigue Syndrome
• Goiter
• Congestive Heart Failure
• Hypopituitarism
• Pituitary tumor
• Addison's disease

Diagnostic Tests for Hypothyroidism

• Primary test is TSH level in a blood sample
• Normal TSH level is 0.4 -4 mIU/L.
• Changes in TSH serve as an early warning system.
• Elevated TSH indicates that the thyroid gland is not producing enough thyroid hormone, indicating primary hypothyroidism.
• T4 is the main form of thyroid hormone circulating in the blood
• The majority of T4 in the blood is attached to thyroxine-binding globulin.
• Total T4 measures both the bound and free hormone.
• Free T4 measures what is available and able to affect body tissue functioning.
• Normal Free T4 level ranges from 0.8 to 1.8 ng/dL.
• Elevated TSH and low FT4 or FTI indicate primary hypothyroidism.
• Lipid profile may show hyperlipidemia
• CBC may show anemia
• ECG may detect bradycardia
• Imaging test is usually not necessary unless there is a nodule

Lab Findings in Hypothyroidism

• Normal Condition: Normal TSH, Normal Free T4
• Primary Hypothyroid: High TSH, Low Free T4
• Subclinical Hypothyroid : High TSH, Normal Free T4
• Hashimoto's: High TSH, Low Free T4, present antibodies (TPO)

Hypothyroidism Treatment

• Prescribing levothyroxine sodium (synthetic thyroxine, T4), such as Synthroid or Levoxyl
  • In young, healthy, non-pregnant adults, administer 1.6 mcg/kg PO daily
  • In adults > 50-60 years or with CHD, administer 25 to 50 mcg PO daily
• In adults > 50-60 years with CHD, administer 12.5 to 25 mcg PO daily.
• Educating clients that they will need lifelong therapy
• Adverse effects of treatment may mimic s/s of hyperthyroidism
• Take medication on an empty stomach (30-60 mins before breakfast or 2-4 hours after last meal)
• Certain medications interfere with absorption (ex. estrogen therapy, antacids, iron)
• Monitor serum TSH 4-8 weeks after initiation of therapy or after dosage adjustments
• Periodic TSH measurements at 6 months & then 12 month intervals, unless symptomatic

Myxedema Coma

• Severe, life-threatening and decompensated hypothyroidism
• Thyroid hormone levels are dangerously low
• The condition is common in elderly women with long-standing preexisting hypothyroidism.
• Cold temperature can be a trigger
• Preexisting hypothyroidism may present with myxedema coma after a period of prolonged noncompliance with thyroid hormone replacement.
• Signs and symptoms- extreme lethargy, can progress to stupor or coma, hypothermia, respiratory depression, bradycardia, hyponatremia, and renal impairment

Hyperthyroidism

• Clinical state when the body's tissues are exposed to an increased level of circulating thyroid hormone.
• Hyperthyroidism's prevalence is approximately 5-10 times less than hypothyroidism.
• White and Hispanic populations in the U.S. have a higher prevalence than black populations.
• Women are more affected than men (7:1).
• Age at diagnosis is predominantly 20-40 years.
• Overt hyperthyroidism does not require the presence of symptoms or nonspecific symptoms.
• There is a low or undetectable TSH level with elevated T4 or T3 level.
• Subclinical hyperthyroidism is an asymptomatic condition.
• There is a low serum TSH level with normal T4 and triiodothyronine (T3) levels.

Common Causes of Hyperthyroidism

• Graves disease
• Functional thyroid nodules
• Toxic multinodular goiter
• Toxic adenoma

Risk Factors for a Low TSH Level

• Female sex
• Advancing age
• Black race
• Low iodine intake
• Personal or family history of thyroid disease
• Ingestion of iodine-containing drugs, such as amiodarone.

Classic Presentation of Hyperthyroidism

• Adrenergic symptoms, hypermetabolism
• Heat intolerance
• Palpitations
• Tremor
• Nervousness
• Insomnia
• Weight loss despite increased appetite
• Hyperdefecation
• Menstrual Abnormalities

Physical Exam findings in Hyperthyroidism

• Warm, moist skin
• Lid lag
• Brisk deep tendon reflexes
• Tachycardia
• Neuromuscular symptoms
• Psychiatric symptoms
• Long-standing untreated hyperthyroidism may develop atrial fibrillation (10% to 15% of patients) or heart failure (5.8% of patients)
• Graves orbitopathy (exophthalmos or periorbital edema)
• Photophobia, excessive lacrimation, increased eye sensitivity to wind or smoke, or a sensation of a foreign body in the eyes

Differential Diagnoses for Hyperthyroidism

• Anxiety
• Arrhythmias
• Diabetes Mellitus
• Malignancy
• Pheochromocytoma
• Depression
• Menopause
• Panic disorder

Diagnostic Tests for Hyperthyroidism

• Low TSH level indicates that the thyroid is producing too much thyroid hormone (hyperthyroidism)
• Free T4 - elevated
• Serum Total T3= 75 to 195 ng/dL (1.1 to 3 nmol/L)
• Measurement of free T3 is doable but is often not dependable
• TSI (thyroid stimulating immunoglobulin) is elevated in Graves disease
• Other labs include Liver Profile; elevate alk phos, ALT, AST, total bilirubin, CBC - ESR elevation
• Ultrasound – to evaluate goiter, nodule
• Radioiodine Uptake Scan - elevated uptake in Graves Disease, toxic nodule
• EKG - tachycardia, afib

Lab Findings in Hyperthyroidism

• Primary Hyperthyroid: Low TSH, High Free T4, High T3
• Subclinical Hyperthyroid: Low TSH, Normal Free T4, Normal T3
• TSI elevated in Graves

Hyperthyroidism Treatment

• Beta-blockers to reduce symptoms associated with hyperthyroidism
  • Propanolol 10-40 mg PO 3-4 times per day
  • Atenolol 25-100 mg PO 1-2 times per day
  • Caution in patients with asthma or CHF
• Antithyroid drugs to decrease thyroid hormone synthesis
  • Methimazole is the 1st line initial 10-30 mg PO daily; and maintenance 5-10 mg PO daily.
  • Propylthiouracil (PTU) 50-100 mg PO TID... Should ONLY be used in the first trimester of pregnancy.

Hyperthyroidism Management

• Referral for Radioactive iodine therapy or surgery to the Thyroid (Thyroidectomy)
• Follow up to Monitor serum TSH, T3 & free T4 4-6 weeks after initiation of thionamides (and after radioactive iodine therapy); then at 3-6-month intervals.

Thyroid Storm

• Decompensated, severe form of hyperthyroidism, with increased morbidity and mortality.
• Elevated serum thyroid hormone concentrations, which causes extreme alteration of usual hyperthyroid symptoms. Diagnosis can occur both in patients with or without preexisting hyperthyroidism.
• Rare diagnosis triggered by acute exposure to excess iodine or from surgery
• Patients with known severe hyperthyroidism may also form thyroid storm.
• Rapid recognition is necessary to begin management in an ICU setting.
• Clinical manifestations include fever, cardiac arrhythmias, vomiting, and impaired mental status.

Geriatric Considerations for Thyroid Disorders

• TSH screening is recommended because of high prevalence of hypothyroidism in women > 60 yrs & the presence of subtle symptoms.
• Elderly less likely to experience thyrotoxicosis than hypothyroidism.
• Most common presentation of hyperthyroidism in patients > 60 yrs is atrial fibrillation.
• In older patients, initiate levothyroxine dose at 25 mcg PO daily & increase gradually to 1.0 mcg/kg/daily
• Watch for treatment-induced angina, CHF, or arrhythmias (particularly atrial fibrillation)
• Post-menopausal women with untreated Graves disease or on thyroid replacement therapy: may require increased dosage of levothyroxine if estrogen therapy is initiated concurrently.
• Important to check bone density because prone to accelerated bone loss & osteoporosis

Explore questions about hypothyroidism, its symptoms like delayed reflexes and dry skin, and lab results including TSH and T4 levels. Learn about TRH release and the role of TSH as an early indicator of thyroid issues. Identify conditions causing hypothyroid symptoms despite normal thyroid function.