class User extends Model {
    public function threads() {
		return $this->belongsToMany(Thread::class)->withPivot('last_read_message_id');
	}
}

class Thread extends Model {
    public function users() {
		 return $this->belongsToMany(User::class)->withPivot('last_read_message_id');
	}

    public function messages() {
		return $this->hasMany(ThreadMessage::class);
	}
}

class ThreadMessage extends Model {
    public function thread() {
		return $this->belongsTo(Thread::class);
	}

    public function author() {
		return $this->belongsTo(User::class);
	}
}

// ...
Auth::user()->threads;