thyroid and steroid hormones on growth

What do thyroid hormones regulate?

Metabolism, thermoregulation, and growth.

What stimulates thyroid hormone production?

TRH (hypothalamus) → TSH (pituitary) → T3/T4 (thyroid).

What happens in hypothyroid children?

Slowed growth, dwarfism, and mental delay (cretinism).

What happens to fetal growth if thyroid hormones are low?

↓ Birthweight, ↓ organ and skeletal size, ↓ protein in tissues.

How does T3/T4 promote growth?

↑ Oxygen consumption, mitochondrial activity, ATP production, and cardiac output.

When does fetal thyroid start working?

2nd trimester.

Can maternal T3/T4 cross the placenta?

Some can in species with a hemochorial placenta (e.g., humans).

What does the placenta actively transport to the fetus for thyroid function?

Iodide (Na⁺/I⁻).

What fetal factors does thyroid hormone rely on for growth effects?

IGF and insulin.

What are steroid hormones made from?

Cholesterol.

How do steroids enter cells?

Freely diffuse across membranes and bind cytoplasmic or nuclear receptors.

What changes steroid bioactivity?

Whether they are free or bound to carrier proteins (CBG, albumin).

What is progesterone’s (P4) role in pregnancy?

Maintains uterine environment and immune tolerance of the fetus.

What does cortisol do in late gestation?

Supports organ maturation and energy supply.

What does maternal stress do to hormone balance?

↑ Cortisol, ↓ progesterone → impaired fetal growth.

How does cortisol affect the liver?

↑ Glycogenolysis and gluconeogenesis.

How does cortisol affect muscle?

↑ Protein catabolism.

How does cortisol affect fat?

↑ Lipolysis (to spare glucose).

When do sex steroids first influence development?

Fetal stage – testosterone and estrogen drive sex-specific development.

What happens postnatally before puberty?

Low sex steroids stimulate some tissue growth.

What happens at puberty?

↑ Sex steroids → secondary sex traits and bone growth plate closure.

What does estrogen do during development?

Promotes anabolic growth at low levels; closes growth plates at puberty.

What tissues are influenced by both estrogen and androgen?

Bone, muscle, fat, reproductive tissues, skin, and brain.

What cells are involved in bone growth?

Chondrocytes (cartilage), osteoblasts (build bone), osteoclasts (resorb bone).

What is the main androgen?

Testosterone (produced by testes).

What weak androgens are produced by adrenals?

DHEA and androstenedione.

What is the mechanism of anabolic steroids?

Bind the androgen receptor (AR) to increase muscle and bone growth.

What are risks of anabolic steroid abuse?

Premature growth plate closure, reproductive suppression, behavior changes.

How does long bone growth occur?

Via endochondral ossification (cartilage → bone).

What ends long bone growth?

Estrogen-induced closure of the epiphyseal growth plate.

Why do females reach puberty and stop growing earlier?

Estrogen levels rise earlier and cause earlier growth plate fusion.

What happens with early estrogen exposure (precocious puberty)?

Early growth plate closure → short adult height.

What happens if estrogen is absent or receptors don’t respond?

Long bone growth continues beyond puberty → excessive adult height.

Why are anabolic steroids used in livestock?

To increase growth efficiency and muscle mass.

How are steroids administered in livestock?

Via implants (ear), feed, or injection.