From the Guidelines
Hypothyroidism occurs in several distinct types, including primary, secondary, tertiary, congenital, and subclinical hypothyroidism, with primary hypothyroidism being the most common form, often caused by Hashimoto's thyroiditis, an autoimmune condition where the immune system attacks the thyroid tissue, as noted in studies such as 1 and 1. The different types of hypothyroidism include:
- Primary hypothyroidism: the most common form, accounting for over 95% of cases, and occurs when the thyroid gland itself fails to function properly
- Secondary hypothyroidism: results from pituitary gland dysfunction, where the pituitary fails to produce enough thyroid-stimulating hormone (TSH) to stimulate the thyroid
- Tertiary hypothyroidism: stems from hypothalamic dysfunction, where the hypothalamus doesn't produce enough thyrotropin-releasing hormone (TRH) to stimulate the pituitary
- Congenital hypothyroidism: occurs when babies are born with an absent or underdeveloped thyroid gland or have inherited enzymatic defects in thyroid hormone production
- Subclinical hypothyroidism: represents a mild form where TSH levels are elevated but thyroid hormone levels remain normal, often progressing to overt hypothyroidism over time These types of hypothyroidism can be caused by various factors, including:
- Hashimoto's thyroiditis, an autoimmune condition where the immune system attacks the thyroid tissue
- Iodine deficiency
- Surgical removal of the thyroid
- Radiation treatment
- Certain medications like lithium or amiodarone
- Pituitary gland dysfunction
- Hypothalamic dysfunction
- Inherited enzymatic defects in thyroid hormone production Treatment typically involves thyroid hormone replacement with levothyroxine (T4), with dosing individualized based on the patient's age, weight, severity of deficiency, and underlying cause, as recommended in studies such as 1 and 1.
From the FDA Drug Label
Thyroid hormone drugs are indicated: As replacement or supplemental therapy in patients with hypothyroidism of any etiology, except transient hypothyroidism during the recovery phase of subacute thyroiditis This category includes cretinism, myxedema and ordinary hypothyroidism in patients of any age (pediatric patients, adults, the elderly), or state (including pregnancy); primary hypothyroidism resulting from functional deficiency, primary atrophy, partial or total absence of thyroid gland, or the effects of surgery, radiation, or drugs, with or without the presence of goiter; and secondary (pituitary) or tertiary (hypothalamic) hypothyroidism Levothyroxine Sodium Tablet Dosing Guidelines for Hypothyroidism in Pregnant Patients Patient Population Starting Dosage Dose Adjustment and Titration Pre-existing primary hypothyroidism with serum TSH above normal trimester- specific range Pre-pregnancy dosage may increase during pregnancy Increase levothyroxine sodium dosage by 12. 5 to 25 mcg per day. Monitor TSH every 4 weeks until a stable dose is reached and serum TSH is within normal trimester-specific range. Reduce levothyroxine sodium dosage to pre-pregnancy levels immediately after delivery. Monitor serum TSH 4 to 8 weeks postpartum. New onset hypothyroidism (TSH ≥10 IU per liter)
- 6 mcg/kg/day Monitor serum TSH every 4 weeks and adjust levothyroxine sodium dosage until serum TSH is within normal trimester-specific range. New onset hypothyroidism (TSH < 10 IU per liter)
- 0 mcg/kg/day
The different types of hypothyroidism include:
- Primary hypothyroidism: resulting from functional deficiency, primary atrophy, partial or total absence of thyroid gland, or the effects of surgery, radiation, or drugs, with or without the presence of goiter
- Secondary (pituitary) hypothyroidism
- Tertiary (hypothalamic) hypothyroidism
- Cretinism
- Myxedema
- Ordinary hypothyroidism
- Transient hypothyroidism during the recovery phase of subacute thyroiditis
- New onset hypothyroidism These types of hypothyroidism can occur due to various etiology, including:
- Functional deficiency
- Primary atrophy
- Partial or total absence of thyroid gland
- Effects of surgery
- Effects of radiation
- Effects of drugs
- Pregnancy 2 3
From the Research
Types of Hypothyroidism
- Primary hypothyroidism: defined as elevated thyroid-stimulating hormone (TSH) concentration in combination with free thyroxine (fT4) concentration below the reference range 4
- Subclinical hypothyroidism: defined by elevated TSH concentrations but fT4 concentrations within the reference range 4, 5
- Central hypothyroidism: caused by a deficiency of thyrotropin-releasing hormone or thyroid-stimulating hormone (TSH) 5
- Peripheral hypothyroidism: caused by a problem with the peripheral tissues 4
- Consumptive hypothyroidism: caused by excessive inactivation of thyroid hormone 5
Causes of Hypothyroidism
- Autoimmune diseases, such as chronic autoimmune thyroiditis (Hashimoto's thyroiditis) 6, 4, 7
- Radiation therapy 6, 7
- Thyroid gland removal surgeries 6
- Certain medications, such as amiodarone, lithium, and immune checkpoint inhibitors 7
- Severe iodine deficiency 4, 7
- Drugs, such as those that affect thyroid function 5